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astro-ph/9504069
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Preprint account
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Faint UBVRI CCD sequences for wide-field surveys - I
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astro-ph
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We present the first results of a campaign to secure deep UBVRI CCD
photometric calibration in all UK Schmidt equatorial fields with galactic
latitudes |b| > 50deg. In this paper we provide information on deep BVRI
sequences (B<22) in 14 UK Schmidt survey fields centred at declination=0deg.
Deep U sequences have been obtained for 11 of these fields.
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astro-ph/0510464
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Kazimierz St\c{e}pie\'n
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Evolutionary status of late-type contact binaries
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astro-ph
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A new scenario for evolution of contact binaries is presented and discussed.
Arguments are given that W UMa-type systems are formed from detached binaries
which lose angular momentum via magnetized wind. This takes typically several
Gyr. Such an age is sufficient for the initially more massive component to
deplete hydrogen in its core and to fill the Roche lobe. As a result, mass
transfer occurs with a reversal of the mass ratio. After a fast mass exchange
the present primaries of W UMa-type binaries land close to ZAMS whereas the
secondaries develop small helium cores, which makes them considerably
over-sized relative to ZAMS stars of the same mass. As a result, both
components can fulfill the mass-radius relation for contact systems while being
in thermal equilibrium. This solves the "Kuiper paradox". Further mass transfer
in a contact phase proceeds on a nuclear time scale of the secondary. Hydrogen
rich matter is transferred to the primary in the first part of this phase. This
keeps the primary close to ZAMS. When the mass of the secondary decreases below
the mass of the original hydrogen-burning core, helium-rich matter is
transferred to the primary shifting it towards TAMS. This prediction agrees
with observations of the position of the components of contact binaries on the
mass-radium diagram. The ultimate fate of the binary is coalescence of both
components.
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astro-ph/0410131
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A. Katherina Vivas
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VLT Spectroscopy of RR Lyrae Stars in the Sagittarius Tidal Stream
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astro-ph
|
Sixteen RR Lyrae variables from the QUEST survey that lie in the leading arm
of the tidal stream from the Sagittarius dSph galaxy have been observed
spectroscopically to measure their radial velocities and metal abundances. The
systemic velocities of 14 stars, which were determined by fitting a standard
velocity curve to the individual measurements, have a sharply peaked
distribution with a mean of 33 km/s and a standard deviation of only 25 km/s.
The [Fe/H] distribution of these stars has a mean of -1.76 and a standard
deviation of 0.22. These measurements are in good agreement with previous ones
from smaller samples of stars. The mean metallicity is consistent with the
age-metallicity relation that is observed in the main body of the Sgr dSph
galaxy.
The radial velocities and the distances from the Sun of these stars are
compared with recent numerical simulations of the Sgr streams that assume
different shapes for the dark matter halo. Models that assume a oblate halo do
not fit the data as well as ones that assume a spherical or a prolate
distribution. However, none of the fits are completely satisfactory. Every
model fails to reproduce the long extent of the stream in right ascension (36
degr) that is seen in the region covered by the QUEST survey. Further modeling
is required to see if this and the other mismatches between theory and
observation can be removed by judicial choices for the model parameters or
instead rule out a class of models.
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astro-ph/0511036
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N. P. F. Lorente
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Specsim: The MIRI Medium Resolution Spectrometer Simulator
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astro-ph
|
MIRI, the Mid-InfraRed Instrument, is one of four instruments being built for
the James Webb Space Telescope. It is being developed jointly between an
European Consortium (21 institutes from 10 countries, under the auspices of
ESA), and the US. MIRI consists of an imager, a coronograph, a low-resolution
spectrograph, and an Integral Field Unit (IFU) Medium Resolution Spectrometer
(MIRI-MRS). The latter will be the first mid-infrared IFU spectrograph, and one
of the first IFUs to be used in a space mission. To give the MIRI community a
preview of the properties of the MIRI-MRS data products before the telescope is
operational, the Specsim tool has been developed to model, in software, the
operation of the spectrometer. Specsim generates synthetic data frames
approximating those which will be taken by the instrument in orbit. The program
models astronomical sources and transforms them into detector frames using the
predicted optical properties of the telescope and MIRI. These frames can then
be used to illustrate and inform a range of operational activities, including
data calibration strategies and the development and testing of the data
reduction software for the MIRI-MRS. Specsim will serve as a means of
communication between the many consortium members by providing a way to easily
illustrate the performance of the spectrometer under different circumstances,
tolerances of components and design scenarios.
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astro-ph/9905288
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Javier Gorosabel Urkia
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Optical and near-infrared observations of the GRB 970616 error box
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astro-ph
|
We report on near-infrared and optical observations of the GRB 970616 error
box and of the X-ray sources discovered by ASCA and ROSAT in the region. No
optical transient was found either within the IPN band or in the X-ray error
boxes, similarly to other bursts, and we suggest that either considerable
intrinsic absorption was present (like GRB 970828) or that the optical
transient displayed a very fast decline (like GRB 980326 and GRB 980519).
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astro-ph/0606539
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Mark R. Krumholz
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Magnetohydronamic Evolution of HII Regions in Molecular Clouds:
Simulation Methodology, Tests, and Uniform Media
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astro-ph
|
We present a method for simulating the evolution of HII regions driven by
point sources of ionizing radiation in magnetohydrodynamic media, implemented
in the three-dimensional Athena MHD code. We compare simulations using our
algorithm to analytic solutions and show that the method passes rigorous tests
of accuracy and convergence. The tests reveal several conditions that an
ionizing radiation-hydrodynamic code must satisfy to reproduce analytic
solutions. As a demonstration of our new method, we present the first
three-dimensional, global simulation of an HII region expanding into a
magnetized gas. The simulation shows that magnetic fields suppress sweeping up
of gas perpendicular to magnetic field lines, leading to small density
contrasts and extremely weak shocks at the leading edge of the HII region's
expanding shell.
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astro-ph/0109390
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Lisa Prato
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Near-Infrared Spectroscopy of Brown Dwarfs: Methane and the Transition
between the L and T Spectral Types
|
astro-ph
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We report the possible identification of weak methane spectral features in
the near-infrared K band in two late L dwarfs, DENIS 0205-11 (L7) and 2MASS
1523+30 (L8). New, high signal-to-noise ratio flux-calibrated spectra, spanning
the wavelength interval 1.10 - 2.35 microns with an average resolving power R =
1800 were obtained using NIRSPEC on the Keck II telescope. Results are reported
and compared for three late L dwarfs (L5, L7, and L8) and two T dwarfs (T1 and
T6). The spectra, which are continuous through the atmospheric absorption
bands, show the development of deep steam bands and the weakening of iron
hydride features through the L dwarfs and the emergence of strong methane bands
in the T dwarfs. A detailed comparison of the K band regions with synthetic
spectra suggests that the weak features seen in the L7 and L8 dwarfs at 2.20
microns are likely to be methane. We see no evidence for methane in the H band.
At the R = 1800 resolution, significant differences are evident between the
spectral signatures of the L8 and the T1, leaving room for additional
transition objects (L9 or T0).
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astro-ph/0605513
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Mario Tafalla
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On the internal structure of starless cores. II. A molecular survey of
L1498 and L1517B
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astro-ph
|
[Abridged] We present a molecular survey of the starless cores L1498 and
L1517B. These cores have been selected for their relative isolation and
close-to-round shape, and they have been observed in a number of lines of 13
molecular species (4 already presented in the first part of this series): CO,
CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a
physical model of core structure and a Monte Carlo radiative transfer code, we
determine for each core a self-consistent set abundances that fits
simultaneously the observed radial profile of integrated intensity and the
emergent spectrum towards the core center (for abundant species, optically thin
isopologues are used). From this work, we find that L1498 and L1517B have
similar abundance patterns, with most species suffering a significant drop
toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S,
HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central
hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the
smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3
are present in the gas phase at the core centers. From the different behavior
of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of
molecular depletion. Comparing our abundance determinations with the
predictions from current chemical models we find order of magnitude
discrepancies. Finally, we show how the ``contribution function'' can be used
to study the formation of line profiles from the different regions of a core.
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astro-ph/0504232
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Thom Janssen
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Observations of the binary pulsar system PSR B1718-19 -- The Role of
Tidal Circularisation
|
astro-ph
|
We present optical and infrared observations taken with the Very Large
Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate
companion of the pulsar, identified earlier in Hubble Space Telescope
observations, has been detected in all three bands, R, I, and J. These
detections allowed us to derive constraints on temperature, radius, and mass,
pointing to a companion that has expanded to a radius between one of a main
sequence star and one at the Roche-limit. We focus on the role of tidal
circularisation in the system, which will have transformed the initially
eccentric orbit expected from formation scenarios into the nearly circular
orbit presently observed. Based on simple energy balance arguments, we are able
to draw a picture of the companion's evolution resulting from the energy
deposition in the star due to circularisation. In this picture, our measurement
of the companion's parameters is consistent with the expected initial
eccentricity. However, with the present understanding of tidal dissipation it
remains difficult to account for the short time in which the system was
circularised.
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0806.2348
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Lidia Oskinova
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X-raying clumped stellar winds
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astro-ph
|
X-ray spectroscopy is a sensitive probe of stellar winds. X-rays originate
from optically thin shock-heated plasma deep inside the wind and propagate
outwards throughout absorbing cool material. Recent analyses of the line ratios
from He-like ions in the X-ray spectra of O-stars highlighted problems with
this general paradigm: the measured line ratios of highest ions are consistent
with the location of the hottest X-ray emitting plasma very close to the base
of the wind, perhaps indicating the presence of a corona, while measurements
from lower ions conform with the wind-embedded shock model. Generally, to
correctly model the emerging X-ray spectra, a detailed knowledge of the cool
wind opacities based on stellar atmosphere models is prerequisite. A nearly
grey stellar wind opacity for the X-rays is deduced from the analyses of
high-resolution X-ray spectra. This indicates that the stellar winds are
strongly clumped. Furthermore, the nearly symmetric shape of X-ray emission
line profiles can be explained if the wind clumps are radially compressed. In
massive binaries the orbital variations of X-ray emission allow to probe the
opacity of the stellar wind; results support the picture of strong wind
clumping. In high-mass X-ray binaries, the stochastic X-ray variability and the
extend of the stellar-wind part photoionized by X-rays provide further strong
evidence that stellar winds consist of dense clumps.
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astro-ph/0011218
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Nicolas Gruel
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Luminous compact galaxies at intermediate redshifts: progenitors of
bulges of massive spirals ?
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astro-ph
|
VLT spectra of 14 luminous compact galaxies (LCGs) reveal strong metallic
absorption line sytems as well as narrow and intense emission lines. Their gas
extinction is found to be large (Av ~ 1.5 mag) leading to an upward revision of
their star formation rate (SFR) to an average value of ~ 40 Mo/yr. Large
extinction values are also supported by the large rate of detection in one
field observed by ISO. Gas metal abundances in LCGs have about half the solar
value. We argue that LCGs are the progenitors of present-day spiral bulges.
LCGs have masses and light concentrations similar to those of present-day
bulges. As in present-day galactic bulges, LCG stars show a wide range of
abundances. Thus, observing LCGs allows us to directly witness an important
stage in the formation of a massive galaxy, the building of the bulge prior to
that of the disk. The gas needed to feed the observed star formation is likely
to be falling in from the outskirts of the galaxy, being tidally pulled out
from interacting companion galaxies. Some LCGs also show evidence for the
beginning of a disk formation. If the above scenario holds for most LCGs, we
estimate that at least 20% of present-day spiral galaxies have formed the bulk
of their stars at relatively recent epochs, during the last 8-9 Gyr, at
redshifts less than ~ 1. Since they are heavily extincted, we predict their IR
luminosities to be relatively large, around Lir = 10^11 Lsun, i.e. near or
slightly below the luminosities of the galaxies detected by ISO in the same
redshift range. Taking into account the integrated IR luminosity of the LCG
galaxy population can lead to a significant upward revision of the cosmic SFR
density in the redshift range from 0.5 to 1.
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astro-ph/0205444
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J. M. Bai
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Radio Luminosities and Classificatory Criteria of BL Lacertae Objects
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astro-ph
|
Using the sample of radio selected BL Lacertae objects (RBLs) and X-ray
selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we
calculated the luminosities of radio, optical and X-ray of each source and made
the statistical analysis among the luminosities at different wave-bands,
broad-band spectral indices from radio to X-ray ($\alpha_{\rm rx}$) and peak
frequencies ($\nu_p$). Our results are as follows: (i) there is a positive
correlation between radio luminosity $L_{\rm r}$ and $\alpha_{\rm rx}$ and a
negative correlation between $L_{\rm r}$ and $\nu_p$. High-energy peak BL Lacs
(HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the
dividing lines are probably those of $\log {L_{\rm r}}=43.25$ (erg/sec) and
$\alpha_{\rm rx}>$(or $\leq $)0.75 for $L_{\rm r}$ - $\alpha_{\rm rx}$ plot and
those of $\log {L_{\rm r}}\leq 43.25$ (erg/sec) and $\log {\nu_p}>14.7$ for the
$L_{\rm r}$ - $\nu_p$ plot; (ii) there is a weak positive correlation between
optical luminosity $L_o$ and $\alpha_{\rm rx}$ and a negatively weak
correlation between $L_{\rm o}$ and $\nu_p$; (iii) there is no correlation
between X-ray luminosity $L_X$ and $\alpha_{\rm rx}$ or between $L_X$ and
$\nu_p$. From our analysis, we find that synchrotron radiation is the main
X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs.
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astro-ph/0607445
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Burkhard Fuchs
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CADIS has seen the Virgo overdensity and parts of the Monoceros and
`Orphan' streams in retrospect
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astro-ph
|
We reanalyze deep star counts in five CADIS fields. The data are presented as
vertical density distributions of stars perpendicular to the Galactic plane. In
three fields the profiles are consistent with each other, while in two fields
significant overdensities of stars are found. The overdensity in one field can
be associated with the Virgo overdensity which can be traced right into the
disk of the Milky Way. Using this detection we estimate the mass of the Virgo
overdensity and show that this is equivalent to the stellar content of a Local
Group dwarf spheroidal galaxy. The overdensity in the second field is more
difficult to associate with a previously known overdensity. We suggest that it
is related both to the Monoceros stream and the recently discovered Orphan
stream.
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astro-ph/9907301
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Amri Wandel
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On the Baldwin Effect in Active Galactic Nuclei: I. The
Continuum-Spectrum - Mass Relationship
|
astro-ph
|
We suggest that the Baldwin Effect is a result of the spectral dependence of
the line-driving ionizing continuum on the black hole mass. We derive a
relationship between the mass of the central black hole and the broad emission
line luminosity in active galactic nuclei (AGN). Assuming the UV spectrum of
AGN is emitted from an optically thick medium we find an expression for the
characteristic energy of the ``UV bump'' in terms of the observable luminosity
and emission-line width. We show empirically and analytically that the bump
energy is anti-correlated with the black-hole mass and with the continuum
luminosity. Our model reproduces the observed inverse correlation between
equivalent width and continuum luminosity, yielding an explanation of the
Baldwin effect from first principles. The model gives a good fit to the Baldwin
Effect of the CIV line for a mean quasar EUV spectrum (Zheng et al. 1997) and
for several model spectra. The model also predicts a correlation between the
strength of the Baldwin Effect (the slope of the equivalent width as a function
of luminosity) and the ionization potential, consistent with recent data.
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astro-ph/0505550
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Dieter Nickeler
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Flux and field line conservation in 3--D nonideal MHD flows: Remarks
about criteria for 3--D reconnection without magnetic neutral points
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astro-ph
|
We make some remarks on reconnection in plasmas and want to present some
calculations related to the problem of finding velocity fields which conserve
magnetic flux or at least magnetic field lines. Hereby we start from views and
definitions of ideal and non-ideal flows on one hand, and of reconnective and
non-reconnective plasma dynamics on the other hand. Our considerations give
additional insights into the discussion on violations of the frozen--in field
concept which started recently with the papers by Baranov & Fahr (2003a;
2003b). We find a correlation between the nonidealness which is given by a
generalized form of the Ohm's law and a general transporting velocity, which is
field line conserving.
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astro-ph/0309789
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Orsola de Marco
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The dual dust chemistry - binarity connection
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astro-ph
|
Accumulating evidence points to a binary nature for the Wolf-Rayet ([WC])
central stars, a group that constitutes about 15% of all central stars of
planetary nebula. From ISO observations, a dual dust chemistry (oxygen- and
carbon-rich) has been shown to be almost exclusively associated with [WC]
central stars, a fact that could be explained by O-rich dust residing in a
disk, while the C-rich dust being more widely distributes. HST/STIS space
resolved spectroscopy of the [WC10] central star CPD-568032, is interpreted as
revealing a dust disk or torus around the central star. This, together with
CPD-568032's variable lightcurve is taken as an indirect indication of
binarity. Finally, we present here, for the first time, preliminary results
from a radial velocity survey of central stars. Out of 18 stars with excellent
data at least 8 are radial velocity variables. If these turn out to be
binaries, it is likely that the central star binary fraction is as high as
about 50%.
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astro-ph/9406050
|
Uros Seljak
|
A Two-Fluid Approximation for Calculating the Cosmic Microwave
Background Anisotropies
|
astro-ph
|
We present a simple, yet accurate approximation for calculating the cosmic
microwave background anisotropy power spectrum in adiabatic models. It consists
of solving for the evolution of a two-fluid model until the epoch of
recombination and then integrating over the sources to obtain the CMB
anisotropy power spectrum. The approximation is useful both for a physical
understanding of CMB anisotropies, as well as for a quantitative analysis of
cosmological models. Comparison with exact calculations shows that the accuracy
is typically better than 20 percent over a large range of angles and
cosmological models, including those with curvature and cosmological constant.
Using this approximation we investigate the dependence of the CMB anisotropies
on the cosmological parameters. We identify six dimensionless parameters that
uniquely determine the anisotropy power spectrum within our approximation. CMB
experiments on different angular scales could in principle provide information
on all these parameters. In particular, mapping of the Doppler peaks would
allow an independent determination of baryon mass density, matter mass density
and Hubble constant.
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astro-ph/0204365
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Roberto Nesci
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Optical and Radio monitoring of S5 1803+74
|
astro-ph
|
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of $\sim$30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.
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astro-ph/9606079
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Antoinette Cowie
|
New Insight on Galaxy Formation and Evolution from Keck Spectroscopy of
the Hawaii Deep Fields
|
astro-ph
|
We present the results of spectroscopic studies with the LRIS spectrograph on
Keck of two of the Hawaii deep survey fields. The 393 objects observed cover an
area of 26.2 square arcmin and constitute a nearly complete sample down to K =
20, I = 23, and B = 24.5. The rest-frame K-band luminosity function and its
evolution with redshift are described. Comparisons are made with other
optically selected (B and I) samples in the literature, and the corresponding
rest-frame B-band luminosity function evolution is presented. The B-band counts
near B = 24 are shown to be a mixture of normal galaxies at modest redshifts
and galaxies undergoing rapid star formation, which have a wide range of masses
and which are spread over the redshift interval from z = 0.2 to beyond z = 1.7.
The luminosity functions, number counts, and color distributions at optical and
IR wavelengths are discussed in terms of a consistent picture of the
star-forming history of the galaxy sample. [OII] emission-line diagnostics or
rest-frame ultra-violet--infrared color information are used in combination
with rest-frame absolute K magnitudes to construct a ``fundamental plane'' in
which the evolution of the global star-formation rate with redshift can be
shown, and we find that the maximum rest-frame K luminosity of galaxies
undergoing rapid star formation has been declining smoothly with decreasing
redshift from a value near L* at z > 1. This smooth decrease in the
characteristic luminosity of galaxies dominated by star formation can
simultaneously account for the high B-band galaxy counts at faint magnitudes
and the redshift distribution at z < 1 in both the B- and K-selected samples.
Finally, the overall K-band light density evolution is discussed as a tracer of
the baryonic mass in stars and compared with the rate of star formation.
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astro-ph/0507293
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Eric W. Greisen
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Representations of spectral coordinates in FITS
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astro-ph
|
Greisen & Calabretta describe a generalized method for specifying the
coordinates of FITS data samples. Following that general method, Calabretta &
Greisen describe detailed conventions for defining celestial coordinates as
they are projected onto a two-dimensional plane. The present paper extends the
discussion to the spectral coordinates of wavelength, frequency, and velocity.
World coordinate functions are defined for spectral axes sampled linearly in
wavelength, frequency, or velocity, linearly in the logarithm of wavelength or
frequency, as projected by ideal dispersing elements, and as specified by a
lookup table.
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astro-ph/0003120
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Dayton Jones
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Space VLBI at Low Frequencies
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astro-ph
|
At sufficiently low frequencies, no ground-based radio array will be able to
produce high resolution images while looking through the ionosphere. A
space-based array will be needed to explore the objects and processes which
dominate the sky at the lowest radio frequencies. An imaging radio
interferometer based on a large number of small, inexpensive satellites would
be able to track solar radio bursts associated with coronal mass ejections out
to the distance of Earth, determine the frequency and duration of early epochs
of nonthermal activity in galaxies, and provide unique information about the
interstellar medium. This would be a "space-space" VLBI mission, as only
baselines between satellites would be used. Angular resolution would be limited
only by interstellar and interplanetary scattering.
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astro-ph/9808113
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Konstantin Zioutas
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Search for energetic cosmic axions utilizing terrestrial/celestial
magnetic fields
|
astro-ph
|
Orbiting $\gamma$-detectors combined with the magnetic field of the Earth or
the Sun can work parasitically as cosmic axion telescopes. The relatively short
field lengths allow the axion-to-photon conversion to be coherent for
$m_{axion} \sim 10^{-4}$ eV, if the axion kinetic energy is above $\sim 500$
keV (Earth's field), or, $\sim 50$ MeV (Sun's field), allowing thus to search
for axions from $e^+e^-$ annihilations, from supernova explosions, etc. With a
detector angular resolution of $\sim 1^o$, a more efficient sky survey for
energetic cosmic axions passing {\it through the Sun} can be performed. Axions
or other axion-like particles might be created by the interaction of the cosmic
radiation with the Sun, similarly to the axion searches in accelerator beam
dump experiments; the enormous cosmic energy combined with the built-in
coherent Primakoff effect might provide a sensitive detection scheme, being out
of reach with accelerators. The axion signal will be an excess in $\gamma$-rays
coming either from a specific celestial place behind the Sun, e.g. the Galactic
Center, or, from any other direction in the sky being associated with a violent
astrophysical event, e.g. a supernova. Earth bound detectors are also of
potential interest. The axion scenario also applies to other stars or binary
systems in the Universe, in particular to those with superstrong magnetic
fields.
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astro-ph/0502259
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Elisabetta de Filippis
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Abell 370: A Cluster with a Pronounced Triaxial Morphology
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astro-ph
|
We here combine Sunyaev-Zel'dovich effect, X-ray observations and
spectroscopic redshifts of member galaxies, to constrain the intrinsic
three-dimensional shape of the galaxy cluster: Abell 370. The cluster turns out
to be strongly elongated along the l.o.s., with two (or more) substructures in
the process of merging. Spectroscopy further suggests that the process must be
taking place at a small angle respect to the l.o.s.
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astro-ph/9711199
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Andrew Ulmer
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Tidal Disruption Eddington Envelopes around Massive Black Holes
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astro-ph
|
Optically-thick envelopes may form following the tidal disruption of a star
by a massive black hole. Such envelopes would reprocess hard radiation from
accretion close to the black hole into the UV and optical bands producing
AGN-luminosity flares with duration ~1 year. We show that due to relativistic
effects, the envelopes are convective. If convection is efficient, then the
structure of the envelopes is similar to that described in previous work;
however, the photospheric radius is shown to be very sensitive to the
luminosity at the envelope base, suggesting that either the envelope collapses
or the envelope expands to a maximum radius at which point a wind may set in.
For an envelope without winds, we find a maximum photospheric radius of ~10^16
cm (i.e. minimum effective temperature ~6,000 K). The evolution of the
envelopes is described based on simple energy arguments.
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astro-ph/0410080
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Joseph Silk
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Feedback and the Initial Mass function
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astro-ph
|
I describe a turbulence-inspired model for the stellar initial mass function
which includes feedback and self-regulation via protostellar outflows. A new
aspect of the model provides predictions of the star formation rate in
molecular clouds and gas complexes. A similar approach is discussed for
self-regulation on kiloparsec scales via supernova input, and an expression is
presented for the global star formation rate that depends on the turbulent
pressure of the interstellar medium.
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astro-ph/0303392
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Dawn K. Erb
|
H-alpha Spectroscopy of Galaxies at z>2: Kinematics and Star Formation
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astro-ph
|
We present near-infrared spectroscopy of H-alpha emission lines in a sample
of 16 star-forming galaxies at redshifts 2.0<z<2.6. Our targets are drawn from
a large sample of galaxies photometrically selected and spectroscopically
confirmed to lie in this redshift range. Six of the galaxies exhibit spatially
extended, tilted H-alpha emission lines; rotation curves for these objects
reach mean velocities of ~150 km/s at radii of ~6 kpc, without corrections for
any observational effects. The velocities and radii give a mean dynamical mass
of M>4e10 M_sun. One-dimensional velocity dispersions for the 16 galaxies range
from ~50 to ~260 km/s, and in cases where we have both virial masses implied by
the velocity dispersions and dynamical masses derived from the spatially
extended emission lines, they are in rough agreement. We compare our kinematic
results to similar measurements made at z~3, and find that both the observed
rotational velocities and velocity dispersions tend to be larger at z~2 than at
z~3. We find a mean SFR_H-alpha of 16 M_sun/yr and an average
SFR_H-alpha/SFR_UV ratio of 2.4, without correcting for extinction. We see
moderate evidence for an inverse correlation between the UV continuum
luminosity and the ratio SFR_H-alpha/SFR_UV, such as might be observed if the
UV-faint galaxies suffered greater extinction. We discuss the effects of dust
and star formation history on the SFRs, and conclude that extinction is the
most likely explanation for the discrepancy between the two SFRs.
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No Label
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No Label
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astro-ph/9503012
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Werner Becker
|
ROSAT HRI Observations of the Crab Pulsar: An Improved Temperature upper
limit for PSR 0531+21
|
astro-ph
|
ROSAT HRI observations have been used to determine an upper limit of the Crab
pulsar surface temperature from the off-pulse count rate. For a neutron star
mass of 1.4 \Mo and a radius of 10 km as well as the standard distance and
interstellar column density, the redshifted temperature upper limit is\/
$T_s^\infty \le 1.55\times 10^6$ K $(3\sigma)$. This is the lowest temperature
upper limit obtained for the Crab pulsar so far. Slightly different values for
$T_s^\infty$ are computed for the various neutron star models available in the
literature, reflecting the difference in the equation of state.
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No Label
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No Label
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astro-ph/9805349
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Henri M. J. Boffin
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Numerical simulations of protostellar encounters I. Star-disc encounters
|
astro-ph
|
It appears that most stars are born in clusters, and that at birth most stars
have circumstellar discs which are comparable in size to the separations
between the stars. Interactions between neighbouring stars and discs are
therefore likely to play a key role in determining disc lifetimes, stellar
masses, and the separations and eccentricities of binary orbits. Such
interactions may also cause fragmentation of the discs, thereby triggering the
formation of additional stars.
We have carried out a series of simulations of disc-star interactions using
an SPH code which treats self-gravity, hydrodynamic and viscous forces. We find
that interactions between discs and stars provide a mechanism for removing
energy from, or adding energy to, the orbits of the stars, and for truncating
the discs. However, capture during such encounters is unlikely to be an
important binary formation mechanism.
A more significant consequence of such encounters is that they can trigger
fragmentation of the disc, via tidally and compressionally induced
gravitational instabilities, leading to the formation of additional stars. When
the disc-spins and stellar orbits are randomly oriented, encounters lead to the
formation of new companions to the original star in 20% of encounters. If most
encounters are prograde and coplanar, as suggested by simulations of
dynamically-triggered star formation, then new companions are formed in
approximately 50% of encounters.
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No Label
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No Label
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astro-ph/0307325
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Masaru Ajiki
|
A Subaru Search for Lyman$\alpha$ Emitters at Redshift 5.7
|
astro-ph
|
We present the results of a survey for Ly$\alpha$ emitters at $z\approx 5.7$
based on optical narrow-band ($\lambda_{\rm c} = 8150$ \AA ~ and $\Delta\lambda
= 120$ \AA), and broad-band ($B$, $R_{\rm C}$, $I_{\rm C}$, and $z^\prime$)
observations of the field surrounding the high redshift quasar, SDSSp
J104433.04$-$012522.2, on the 8.2 m Subaru Telescope with the Subaru Prime
Focus Camera, Suprime-Cam. This survey covers a sky area of $\approx 720$
arcmin$^2$ and a co-moving volume of $\simeq 2 \times 10^5 ~ h_{0.7}^{-3}$
Mpc$^3$. We have found 20 candidates of Ly$\alpha$ emitters at $z \approx$ 5.7
with $\Delta z \approx 0.1$. Two of them have been confirmed star-forming
galaxies at $z=5.655$ and $z=5.687$ from our follow-up optical spectroscopy. We
discuss star-formation properties of the 20 objects from a statistical point of
view. Our survey leads to a new estimate of the star formation rate density at
$z \approx 5.7$, $\sim 1.2 \times 10^{-3} h_{0.7} M_\odot$ yr$^{-1}$
Mpc$^{-3}$.
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No Label
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No Label
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0809.4259
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Qi Guo
|
High Redshift Galaxy Populations and their Descendants
|
astro-ph
|
We study model predictions for three high-redshift galaxy populations: Lyman
break galaxies at z~3 (LBGs), optically selected star-forming galaxies at z~2
(BXs), and distant red galaxies at z~2 (DRGs).Our galaxy formation model
simultaneously reproduces the abundances, redshift distributions and clustering
of all three observed populations. The star formation rates (SFRs) of model
LBGs and BXs are lower than those quoted for real samples, reflecting different
initial mass functions and scatter in model dust properties. About 85% of model
galaxies selected as DRGs are star-forming, with SFRs ranging up to 100
M_sun/yr. Model LBGs, BXs and DRGs together account for less than half of all
star formation over the range 1.5<z<3.2. Model BXs have metallicities which
agree roughly with observation, but model LBGs are only slightly more
metal-poor, in disagreement with recent observational results. Model galaxies
are predominantly disk-dominated. About 30% of model galaxies with
M>10^{11}M_sun are classified as LBGs or BXs at the relevant redshifts, while
65% are classified as DRGs. Almost all model LBGs and BXs are central galaxies,
but about a quarter of DRGs are satellites. Half of all LBG descendants at z=2
would be identified as BX's, but very few as DRGs. Clustering increases with
decreasing redshift for descendants of all three populations, becoming stronger
than that of L^* galaxies by z=0, when many have become satellite galaxies.
Their stellar mass growth is dominated by star formation until z~1 and
thereafter by mergers. Most LBGs and DRGs end up as red ellipticals, while BXs
have a more varied fate. 99% of local galaxies with M>10^{11.5}M_sun are
predicted to have at least one LBG/BX/DRG progenitor, and over 70% above
10^{11}M_sun. (abbreviated)
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No Label
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No Label
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astro-ph/0502239
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Gavin Rowell
|
Upper limits to the SN1006 multi-TeV gamma-ray flux from H.E.S.S.
observations
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astro-ph
|
Observations of the shell-type supernova remnant SN1006 have been carried out
with the H.E.S.S. system of Cherenkov telescopes during 2003 (18.2h with two
operating telescopes) and 2004 (6.3h with all four telescopes). No evidence for
TeV gamma-ray emission from any compact or extended region associated with the
remnant is seen and resulting upper limits at the 99.9% confidence level are up
to a factor 10 lower than previously-published fluxes from CANGAROO. For SN1006
at its current epoch of evolution we give limits for a number of important
global parameters. Upper limits on the gamma-ray luminosity (for E=0.26 to 10
TeV, distance d=2 kpc) of L_gamma < 1.7x10^{33} erg s^{-1}, and the total
energy in corresponding accelerated protons, W_p<1.6\times 10^{50} erg (for
proton energies E_p \sim 1.5 to 60 TeV and assuming the lowest value n=0.05
cm^{-3} of the ambient target density discussed in literature) are estimated.
Extending this estimate to cover the range of proton energies observed in the
cosmic ray spectrum up to the knee (we take here E_p ~ 1 GeV to 3 PeV, assuming
a differential particle index -2) gives W_p<6.3x10^{50} erg. A lower limit on
the post-shock magnetic field of B>25microG results when considering the
synchrotron/inverse-Compton framework for the observed X-ray flux and gamma-ray
upper limits.
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No Label
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No Label
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astro-ph/9502088
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Alejandro Gonzalez
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On the Morphology of Density Perturbations
|
astro-ph
|
The morphological distribution of primordial density peaks is assessed.
Previous determinations, those of Peacock & Heavens (1985) and those of Bardeen
et al (1986) have contradictory concluded that there exist a tendency towards
prolate or towards oblate shapes, respectively. By using two methods, the
Hessian and the inertia tensor momenta we have performed numerical
determinations of the triaxiality of density perturbations in Gaussian random
fields with power law spectra. We show that there is no present any tendency of
shapes, and that the triaxiality distribution is independent of the spectral
index. Moreover, it is shown that the results of Peacock and Heavens are
compatible with our determinations. These results are in complete agreement
with current triaxial distributions inferred for galaxies and clusters both,
from observations and from numerical simulations.
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No Label
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astro-ph/9802347
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Brenda L. Frye
|
BIMA and Keck Imaging of the Radio Ring PKS 1830-211
|
astro-ph
|
We discuss BIMA (Berkeley Illinois Maryland Association) data and present new
high quality optical and near-IR Keck images of the bright radio ring PKS
1830-211. Applying a powerful new deconvolution algorithm we have been able to
identify both images of the radio source. In addition we recover an extended
source in the optical, consistent with the expected location of the lensing
galaxy. The source counterparts are very red, I-K=7, suggesting strong Galactic
absorption with additional absorption by the lensing galaxy at z=0.885, and
consistent with the detection of high redshift molecules in the lens.
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No Label
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No Label
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0709.0029
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Judith G. Cohen
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New Extremely Metal-Poor Stars in the Galactic Halo
|
astro-ph
|
We present a detailed abundance analysis based on high resolution and high
signal-to-noise spectra of eight extremely metal poor (EMP) stars with [Fe/H] <
-3.5$ dex, four of which are new. Only stars with 4900 < Teff< 5650 K are
included. Two stars of the eight are outliers in each of several abundance
ratios. The most metal poor star in this sample, HE1424-0241, has [Fe/H] ~ -4
dex and is thus among the most metal poor stars known in the Galaxy. It has
highly anomalous abundance ratios unlike those of any other known EMP giant,
with very low Si, Ca and Ti relative to Fe, and enhanced Mn and Co, again
relative to Fe. Only (low) upper limits for C and N can be derived from the
non-detection of the CH and NH molecular bands. HE0132$-$2429, another sample
star, has excesses of N and Sc with respect to Fe. The strong outliers in
abundance ratios among the Fe-peak elements in these C-normal stars, not found
at somewhat higher metallicities, are definitely real. They suggest that at
such low metallicities we are beginning to see the anticipated and long sought
stochastic effects of individual supernova events contributing to the Fe-peak
material within a single star. A detailed comparison of the results of the
analysis procedures adopted by our 0Z project compared to those of the First
Stars VLT Large Project finds a systematic difference for [Fe/H] of ~0.3 dex,
our values always being higher.
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No Label
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No Label
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astro-ph/0108512
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Denis Puy
|
Primordial molecules at millimeter wavelengths
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astro-ph
|
Chemistry plays a particular role in astrophysics. After atomic hydrogen,
helium and their ions, the Universe probably contains more mass in molecules
than in any other species. Molecule formation in the early, pre-galactic
Universe may have had much to do with the formation of galaxies themselves.
In this context the possible interaction between primordial molecules and
photons of the Cosmic Microwave Background is very important through the
theoretical perspectives and constraints which could give some information on
the theory of the large scale structure formation. In this paper we recall the
more recent progresses on the chemistry of the early Universe, and describe the
importance of molecules in the formation phase of proto objects. A special
attention is done concerning the {\it case of LiH.
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No Label
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No Label
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astro-ph/9606074
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Alice Harding
|
Cylotron line models for the X-ray pulsar A0535+26
|
astro-ph
|
The spectrum of the transient X-ray binary pulsar A0535+26 obtained by OSSE
in February 1994 shows an absorption feature at 110 keV but does not confirm a
feature at around 55 keV, as previously reported by other instruments. If the
110 keV feature is due to cyclotron scattering at the first harmonic, then the
magnetic field required is about $10^{13}$ Gauss, the highest observed in an
X-ray pulsar. Conversely, if this strong feature is a second harmonic and the
line formation process is such that an extremely weak fundamental results at
$\simeq$ 55 keV, the estimate of the field strength is halved. We present
results of detailed cyclotron line transfer models from two source geometries
to explore the theoretical contraints on the line shapes in this source. It is
found that while a fundamental harmonic line at $\sim$ 55 keV may be partially
filled-in by angle redistribution in cylindrical geometries, the required
viewing angles give a second harmonic line shape inconsistent with the
observation. Interpretation of the feature at 110 keV as a first harmonic seen
at small angles to the field yields more consistent line shapes.
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No Label
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No Label
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astro-ph/0204215
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William Cochran
|
Searching for Planets in the Hyades. I. The Keck Radial Velocity Survey
|
astro-ph
|
We describe a high-precision radial velocity search for jovian-mass
companions to main sequence stars in the Hyades star cluster. The Hyades
provides an extremely well controlled sample of stars of the same age, the same
metallicity, and a common birth and early dynamical environment. This sample
allows us to explore the dependence of the process of planet formation on only
a single independent variable: the stellar mass. In this paper we describe the
survey and summarize results for the first five years.
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No Label
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No Label
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astro-ph/0310335
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John S. Arabadjis
|
Small-Scale structure in the Galactic ISM: Implications for Galaxy
Cluster Studies
|
astro-ph
|
Observations of extragalactic objects need to be corrected for Galactic
absorption and this is often accomplished by using the measured 21 cm HI
column. However, within the beam of the radio telescope there are variations in
the HI column that can have important effects in interpreting absorption line
studies and X-ray spectra at the softest energies. We examine the HI and
DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for
HI variations in of up to a factor of three in 1 degree fields. Column density
enhancements would preferentially absorb soft X-rays in spatially extended
objects and we find evidence for this effect in the ROSAT PSPC observations of
two bright clusters of galaxies, Abell 119 and Abell 2142.
For clusters of galaxies, the failure to include column density fluctuations
will lead to systematically incorrect fits to the X-ray data in the sense that
there will appear to be a very soft X-ray excess. This may be one cause of the
soft X-ray excess in clusters, since the magnitude of the effect is comparable
to the observed values.
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No Label
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No Label
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astro-ph/0504545
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Mariano Moles
|
The ALHAMBRA Survey: For a systematic Study of Cosmic Evolution
|
astro-ph
|
ALHAMBRA is a project to gather data to sample a fraction of the Universe
with enough precision to follow the evolution of its content and properties
with z, a kind of Cosmic Tomography. It is defined as a 4 square degrees
photometric survey with 20 contiguous, equal width, medium band filters
covering from 3500 to 9700 A, plus the JHKs NIR bands. It was optimized to get
(for a fixed observing time) the maximum number of objects with accurate
classification and redshift and to be sensitive to relatively faint emission
features. The observations will be carried out with the 3.5m telescope in Calar
Alto (Spain). We intend to reach the limit AB=25 in all the optical filters
from the bluest to 8300 A, and from 24.7 to 23.4 for the remainder. The
expected limit in the NIR is fixed at Ks=20, H=21, J=22. The homogeneous and
contiguous spectral coverage will result in several hundred thousand objects
with accurate SED identification and z-values. The accuracy of the survey will
allow to make detailed studies, including galactic, without the need for any
further follow-up. Indeed, it will provide exciting targets for large
telescopes, the GTC in particular.
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No Label
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astro-ph/0510098
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Kristian Pedersen
|
The host galaxy cluster of the short gamma-ray burst GRB 050509B
|
astro-ph
|
The first arcsecond localization of a short gamma-ray burst, GRB 050509B, has
enabled detailed studies of a short burst environment. We here report on
studies of the environment of GRB 050509B using the Swift X-ray Telescope
(XRT). The XRT error circle of the burst overlaps with an elliptical galaxy in
the cluster of galaxies ZwCl 1234.0+02916. Based on the measured X-ray flux of
the cluster we estimate that the probability for a chance superposition of GRB
050509B and a cluster at least as X-ray bright as this cluster is $< 2\times
10^{-3}$, presenting the first strong case of a short burst located in a
cluster of galaxies. We also consider the case for GRB 050509B being located
behind ZwCl 1234.0+02916 and gravitationally lensed. From the velocity
dispersion of the elliptical galaxy and the temperature of hot intracluster
gas, we model the mass distribution in the elliptical galaxy and the cluster,
and calculate the gravitational lensing magnification within the XRT error
circle. We find that, if GRB050509B would be positioned significantly behind
the cluster, it is most likely magnified by a factor less than two, but that
the burst could be strongly lensed if it is positioned within 2 arcsec of the
center of the bright elliptical galaxy. Further mapping of arcsecond size short
burst error boxes is a new promising route to determine the spatial
distribution of old stars throughout the Universe.
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No Label
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No Label
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astro-ph/0703456
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Brant Robertson
|
Photometric Properties of the Most Massive High-Redshift Galaxies
|
astro-ph
|
We calculate the observable properties of the most massive high-redshift
galaxies in the hierarchical formation scenario where stellar spheroid and
supermassive black hole growth are fueled by gas-rich mergers. Combining
high-resolution hydrodynamical simulations of the hierarchical formation of a
z~6 quasar, stellar population synthesis models, template AGN spectra,
prescriptions for interstellar and intergalactic absorption, and the response
of modern telescopes, the photometric evolution of galaxies destined to host
z~6 quasars are modeled at redshifts z~4-14. These massive galaxies, with
enormous stellar masses of M_star ~10^11.5-10^12 M_sun. and star formation
rates of SFR~10^3-10^4 M_sun yr^-1 at z>~7, satisfy a variety of photometric
selection criteria based on Lyman-break techniques including V-band dropouts at
z>~5, i-band dropouts at z>~6, and z-band dropouts at z>~7. The observability
of the most massive high-redshift galaxies is assessed and compared with a wide
range of existing and future photometric surveys including SDSS, GOODS/HUDF,
NOAO WDFS, UKIDSS, the IRAC Shallow Survey, Pan-STARRS, LSST, and SNAP. Massive
stellar spheroids descended from z~6 quasars will likely be detected at z~4 by
existing surveys, but owing to their low number densities the discovery of
quasar progenitor galaxies at z>7 will likely require future surveys of large
portions of the sky (>~0.5%) at wavelengths lambda>1 micron. The detection of
rare, star-bursting, massive galaxies at redshifts z>~6 would provide support
for the hierarchical formation of the earliest quasars and characterize the
primitive star-formation histories of the most luminous elliptical galaxies.
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No Label
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astro-ph/0611143
|
Katsuaki Asano
|
Energy and Momentum Transfer via Coulomb Frictions in Relativistic Two
Fluids
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astro-ph
|
We numerically calculate the energy and momentum transfer rates due to
Coulomb scattering between two fluids moving with a relative velocity. The
results are fitted by simple functions. The fitting formulae are useful to
simulate outflows from active galactic nuclei and compact high energy sources.
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No Label
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No Label
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astro-ph/0205113
|
Moshe Carmeli
|
Five-Dimrnsional Cosmological Theory of Unified Space, Time and Velocity
|
astro-ph
|
A five-dimensional cosmological theory of gravitation that unifies space,
time and velocity is presented
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No Label
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No Label
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astro-ph/0305277
|
Adam Burrows
|
A Theory for the Radius of the Transiting Giant Planet HD 209458b
|
astro-ph
|
Using a full frequency-dependent atmosphere code that can incorporate
irradiation by a central primary star, we calculate self-consistent boundary
conditions for the evolution of the radius of the transiting planet HD 209458b.
Using a well-tested extrasolar giant planet evolutionary code, we then
calculate the behavior of this planet's radius with age. The measured radius is
in fact a transit radius that resides high in HD 209458b's inflated atmosphere.
Using our derived atmospheric and interior structures, we find that irradiation
plus the proper interpretation of the transit radius can yield a theoretical
radius that is within the measured error bars. We conclude that if HD 209458b's
true transit radius is at the lower end of the measured range, an extra source
of core heating power is not necessary to explain the transit observations.
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No Label
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No Label
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astro-ph/0201279
|
Mario A. Hamuy
|
Type II Supernovae as Standardized Candles
|
astro-ph
|
We present evidence for a correlation between expansion velocities of the
ejecta of Type II plateau supernovae and their bolometric luminosities during
the plateau phase. This correlation permits one to standardize the candles and
decrease the scatter in the Hubble diagram from ~1 mag to a level of 0.4 and
0.3 mag in the V and I bands, respectively. When we restrict the sample to the
eight objects which are well in the Hubble flow (cz > 3,000 km/s) the scatter
drops even further to only 0.2 mag (or 9% in distance), which is comparable to
the precision yielded by Type Ia supernovae and far better than the ``expanding
photosphere method'' applied to Type II supernovae. Using SN 1987A to calibrate
the Hubble diagrams we get Ho=55+/-12.
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No Label
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No Label
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astro-ph/0610176
|
Diana Londish
|
The 2dF BL Lac Survey II
|
astro-ph
|
We report on our further analysis of the expanded and revised sample of
potential BL Lac objects (the 2BL) optically identified from two catalogues of
blue-selected (UV excess) point sources, the 2dF and 6dF QSO Redshift Surveys
(2QZ and 6QZ). The 2BL comprises 52 objects with no apparent proper motion,
over the magnitude range 16.0 < bj< 20.0. Follow-up high signal-to-noise
spectra of 36 2BL objects and NIR imaging of 18 objects, together with data for
19 2BL objects found in the Sloan Digital Sky survey (SDSS), show 17 objects to
be stellar, while a further 16 objects have evidence of weak, broad emission
features, although for at least one of these the continuum level has clearly
varied. Classification of three objects remains uncertain,with NIR results
indicating a marked reduction in flux as compared to SDSS optical magnitudes.
Seven objects have neither high signal-to-noise spectra nor NIR imaging. Deep
radio observations of 26 2BL objects at the VLA resulted in only three further
radio-detections, however none of the three is classed as a featureless
continuum object. Seven 2BL objects with a radio detection are confirmed as
candidate BL Lac objects while one extragalactic (z=0.494) continuum object is
undetected at radio frequencies. One further radio-undetected object is also a
potential BL Lac candidate. However it would appear that there is no
significant population of radio-quiet BL Lac objects.
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No Label
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No Label
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astro-ph/0507143
|
Mark Lacy
|
The Infrared Array Camera component of the Spitzer Space Telescope
Extragalactic First Look Survey
|
astro-ph
|
We present Infrared Array Camera (IRAC) data and source catalogs from the
Spitzer Space Telescope Extragalactic First Look Survey. The data were taken in
four broad bands centered at nominal wavelengths of 3.6, 4.5, 5.8 and 8.0
microns. A set of mosaics and catalogs have been produced which are ~80%
complete and ~99% reliable to their chosen flux density limits. The main field
survey covers 3.8 deg^2, and has flux density limits of 20muJy, 25muJy, 100muJy
and 100muJy at wavelengths of 3.6, 4.5, 5.8 and 8.0 microns,respectively. The
deeper ``verification'' survey covers 0.25 deg^2 with limits of 10muJy, 10muJy,
30muJy and 30muJy, respectively. We also include deep data in the ELAIS-N1
field which covers 0.041deg^2 with limits of 4muJy, 3muJy, 10muJy and 10muJy,
respectively, but with only two wavelength coverage at a given sky position.
The final bandmerged catalogs contain 103193 objects in the main field, 12224
in the verification field and 5239 in ELAIS-N1. Flux densities of high
signal-to-noise objects are accurate to about 10%, and the residual systematic
error in the absolute flux density scale is ~2-3%. We have successfully
extracted sources at source densities as high as 100000 deg^-2 in our deepest
3.6 and 4.5 micron data. The mosaics and source catalogs will be made available
through the Spitzer Science Center archive and the Infrared Science Archive.
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No Label
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No Label
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astro-ph/0611260
|
Evgeny Derishev
|
Synchrotron emission in the fast cooling regime: which spectra can be
explained?
|
astro-ph
|
We consider the synchrotron emission from relativistic shocks assuming that
the radiating electrons cool rapidly (either through synchrotron or any other
radiation mechanism). It is shown that the theory of synchrotron emission in
the fast cooling regime can account for a wide range of spectral shapes. In
particular, the magnetic field, which decays behind the shock front, brings
enough flexibility to the theory to explain the majority of gamma-ray burst
spectra even in the parameter-free fast cooling regime. Also, we discuss
whether location of the peak in observed spectral energy distributions of
gamma-ray bursts and active galactic nuclei can be made consistent with
predictions of diffusive shock acceleration theory, and find that the answer is
negative. This result is a strong indication that a particle injection
mechanism, other than the standard shock acceleration, works in relativistic
shocks.
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No Label
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No Label
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astro-ph/0103190
|
Claire J. Chandler
|
Sub-arcsecond imaging of SiO in the HH 211 protostellar jet
|
astro-ph
|
We present images of the HH 211 molecular jet in the SiO v=0, J=1-0 line at
43 GHz made with the Very Large Array at approximately 0.5 arcsec resolution.
The SiO emission appears to trace primarily internal bowshocks in the outflow,
suggesting that the dust and molecular gas are accelerated via prompt
entrainment at internal working surfaces in the jet. There is also some
evidence for limb-brightening of the SiO emission, indicating that SiO emission
may also arise from entrainment in the jet's boundary layer. Excitation
temperatures of >~150-200 K are inferred from the SiO emission. Enhancements in
the SiO abundance of ~10^6 over interstellar values are observed, and the
possible origin of the SiO is discussed.
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No Label
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No Label
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astro-ph/0407327
|
Michael G. Richer
|
Planetary Nebulae as Probes of the Chemical Evolution of Dynamically Hot
Systems
|
astro-ph
|
The measurement of chemical abundances in planetary nebulae in nearby
galaxies is now relatively straightforward. The challenge is to use these
chemical abundances to infer the chemical evolution of their host galaxies. At
this point, our understanding of chemical evolution based upon planetary
nebulae in galaxies without star formation is strongly coupled to our
understanding of the relationship between the chemical abundances in stars and
planetary nebulae in the Milky Way bulge. Supposing that the same relation
holds in all systems where star formation ceased long ago, these systems follow
a metallicity-luminosity relation that is displaced to higher chemical
abundances compared to that found for dwarf irregular galaxies. A more
efficient enrichment process appears to be required as part of the explanation
for this shift, in addition to the inevitable fading of these galaxies.
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No Label
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No Label
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astro-ph/0401200
|
Alessandro Omizzolo
|
Digitatio and Scientific Exploitation of the Italian and Vatican
Astronomical Plate Archives
|
astro-ph
|
There is a widespread interest to digitize the precious information contained
in the astronomical plate archives, both for the preservation of their content
and for its fast distribution to all interested researchers in order to achieve
their better scientific exploitation. This paper presents the first results of
our large-scale project to digitize the archive of plates of the Italian
Astronomical Observatories and of the Specola Vaticana. Similar systems,
composed by commercial flat-bed retro-illuminated scanners plus dedicated
personal computers and acquisition and analysis software, have been installed
in all participating Institutes. Ad-hoc codes have been developed to acquire
the data, to test the suitability of the machines to our scientific needs, and
to reduce the digital data in order to extract the astrometric, photometric and
spectroscopic content. Two more elements complete the overall project: the
provision of high quality BVRI CCD sequences in selected fields with the Campo
Imperatore telescopes, and the distribution of the digitized information to all
interested researchers via the Web. The methods we have derived in the course
of this project have been already applied successfully to plates taken by other
Observatories, for instance at Byurakan and at Hamburg.
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astro-ph/9801291
|
Andrew Jaffe
|
The Effect of the Detector Response Time on Bolometric Cosmic Microwave
Background Anisotropy Experiments
|
astro-ph
|
We analyze the effects of the detector response time on bolometric
measurements of the anisotropy of the cosmic microwave background (CMB). We
quantify the effect in terms of a single dimensionless parameter $L$ defined as
the ratio between the time the beam sweeps its own size and the bolometer
response time. As $L$ decreases below $\sim 2.5$ the point source response of
the experiment becomes elongated. We introduce a window function matrix based
on the timestream data to assess the effects of the elongated beam. We find
that the values of the window function matrix elements decrease slowly as a
function $L$. Our analysis and results apply to other cases of beam asymmetry.
For the High Frequency Instrument on board the Planck Surveyor satellite we
show that for a broad range of $L$ the ability of the experiment to extract the
cosmological parameters is not degraded. Our analysis enhances the flexibility
in tuning the design parameters of CMB anisotropy experiments.
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astro-ph/9603156
|
Mike Owen
|
Baryons, Dark Matter, and the Jeans Mass in Simulations of Cosmological
Structure Formation
|
astro-ph
|
We investigate the properties of hybrid gravitational/hydrodynamical
simulations, examining both the numerics and the general physical properties of
gravitationally driven, hierarchical collapse in a mixed baryonic/dark matter
fluid. We demonstrate that, under certain restrictions, such simulations
converge with increasing resolution to a consistent solution. The dark matter
achieves convergence provided that the relevant scales dominating nonlinear
collapse are resolved. If the gas has a minimum temperature (as expected when
intergalactic gas is heated by photoionization due to the ultraviolet
background) and the corresponding Jeans mass is resolved, then the baryons also
converge. However, if there is no minimum baryonic collapse mass or if this
scale is not resolved, then the baryon results err in a systematic fashion. In
such a case, as resolution is increased the baryon distribution tends toward a
higher density, more tightly bound state. We attribute this to the fact that
under hierarchical structure formation on all scales there is always an earlier
generation of smaller scale collapses, causing shocks which irreversibly alter
the state of the baryon gas. In a simulation with finite resolution we miss
such earlier generation collapses, unless a physical scale is introduced below
which structure formation is suppressed in the baryons. We also find that the
baryon/dark matter ratio follows a characteristic pattern, such that collapsed
structures possess a baryon enriched core (enriched by factors of 2 or more
over the universal average) which is embedded within a dark matter halo, even
without accounting for radiative cooling of the gas. The dark matter is
unaffected by changing the baryon distribution (at least in the dark matter
dominated case investigated here).
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astro-ph/9906089
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Coel Hellier
|
The SW Sextantis stars
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astro-ph
|
I review the observational properties of SW Sex stars. I show that they can
be explained by an accretion stream overflowing the disc, combined with an
accretion disc wind. I suggest that SW Sex behaviour is caused by episodes of
very high mass transfer, which are balanced by VY Scl low states.
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0806.2060
|
Gabriele Breuer
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Magnetic fields and star formation in spiral galaxies
|
astro-ph
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The main observational results from radio continuum and polarization
observations about the magnetic field strength and large-scale pattern for
face-on and edge-on spiral galaxies are summarized and compared within our
sample of galaxies of different morphological types, inclinations, and star
formation rates (SFR). We found that galaxies with low SFR have higher thermal
fractions/smaller synchrotron fractions than those with normal or high SFR.
Adopting an equipartition model, we conclude that the nonthermal radio emission
and the \emph{total magnetic field} strength grow nonlinearly with SFR, while
the regular magnetic field strength does not seem to depend on SFR. We also
studied the magnetic field structure and disk thicknesses in highly inclined
(edge-on) galaxies. We found in four galaxies that - despite their different
radio appearance - the vertical scale heights for both, the thin and thick
disk/halo, are about equal (0.3/1.8 kpc at 4.75 GHz), independently of their
different SFR. This implies that all these galaxies host a galactic wind, in
which the bulk velocity of the cosmic rays (CR) is determined by the total
field strength within the galactic disk. The galaxies in our sample also show a
similar large-scale magnetic field configuration, parallel to the midplane and
X-shaped further away from the disk plane, independent of Hubble type and SFR
in the disk. Hence we conclude that also the large-scale magnetic field pattern
does not depend on the amount of SFR.
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0806.0627
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Gregory Sivakoff
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Measurements of Variability of Low Mass X-ray Binary Candidates in the
Early-Type Galaxy NGC 4697 from Multi-Epoch Chandra X-ray Observations
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astro-ph
|
Multi-epoch Chandra X-ray observations of nearby massive early-type galaxies
open up the study of an important regime of low-mass X-ray binary (LMXB)
behavior -- long term variability. In a companion paper, we report on the
detection of 158 X-ray sources down to a detection/completeness limit of
0.6/1.4 x 10^{37} ergs/s using five Chandra observations of NGC 4697, one of
the nearest (11.3 Mpc), optically luminous (M_B < -20), elliptical (E6) galaxy.
In this paper, we report on the variability of LMXB candidates measured on
timescales from seconds to years. At timescales of seconds to hours, we detect
five sources with significant variability. Approximately 7% of sources show
variability between any two observations, and 16+/-4% of sources do not have a
constant luminosity over all five observations. Among variable sources, we
identify eleven transient candidates, with which we estimate that if all LMXBs
in NGC 4697 are long-term transients then they are on for ~ 100 yr and have a
7% duty cycle. These numbers are consistent with those found for brighter LMXBs
in M87 and NGC 1399, which suggests that there does not appear to be a
measurable difference between the outburst durations of long-term transient
neutron star LMXBs and black hole LMXBs. We discuss in detail a transient
supersoft source, whose properties are not easily explained by standard
explanations for supersoft sources.
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astro-ph/9701216
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A. Kashlinsky
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Limits on the cosmic infrared background from clustering in COBE/DIRBE
maps
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astro-ph
|
We discuss a new method of estimating the cosmic infrared background (CIB)
from the spatial properties of infrared maps and give the limits on the CIB
from applying it to the COBE/DIRBE maps. The strongest limits are obtained at
mid- to far-IR where foregrounds are bright, but smooth. If the CIB comes from
matter clustered like galaxies, the smoothness of the maps implies CIB levels
less than $\sim$(10-15) nW/m$^2$/sr over this wavelength range.
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astro-ph/9611004
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Ian Smail
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HST Observations of Distant Clusters: Implications for Galaxy Evolution
|
astro-ph
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The ``MORPHS'' group has completed the cataloging, parameterization, and
morphological classification of ~2000 galaxies in 10 rich clusters from 0.36 <
z < 0.56. From a weak lensing analysis using these data, which compares the
X-ray properties (L_X) of the clusters with virial temperature estimates (T_v)
from the lensing shear strength, we find little evidence for evolution in the
L_X-T_v relation from that observed for local clusters. We discuss how this
observation constrains models for the X-ray evolution of clusters. The data
have also been used to study the color dispersion of bona-fide ellipticals in
high-z clusters: we find the spread to be very small, suggesting an early
formation epoch for the stellar populations of cluster ellipticals. This is
consistent with the evolution of the morphology-density relationship, in which
we find ellipticals to be as abundant at z=0.5 as in clusters today, and
already well ensconced in the dense regions. In contrast, S0's are less
plentiful and less well-concentrated compared to the present epoch, and spiral
galaxies everywhere more abundant. Combined with other spectroscopic and
morphological data, these observations suggest that most of these rapidly
evolving systems are not likely to become bright ellipticals, which were more
likely formed at early epochs. Cluster S0 galaxies, on the other hand, are
likely to have been produced in large numbers in the recent past.
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astro-ph/0105401
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Kazuhiro Shimasaku
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Statistical Properties of Bright Galaxies in the SDSS Photometric System
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astro-ph
|
We investigate the photometric properties of 456 bright galaxies using
imaging data recorded during the commissioning phase of the Sloan Digital Sky
Survey (SDSS). Morphological classification is carried out by correlating
results of several human classifiers. Our purpose is to examine the statistical
properties of color indices, scale lengths, and concentration indices as
functions of morphology for the SDSS photometric system. We find that $u'-g'$,
$g'-r'$, and $r'-i'$ colors of SDSS galaxies match well with those expected
from the synthetic calculation of spectroscopic energy distribution of template
galaxies and with those transformed from $UBVR_CI_C$ color data of nearby
galaxies. The agreement is somewhat poor, however, for $i'-z'$ color band with
a discrepancy of $0.1-0.2$ mag. With the aid of the relation between surface
brightness and radius obtained by Kent (1985), we estimate the averages of the
effective radius of early type galaxies and the scale length of exponential
disks both to be 2.6 kpc for $L^*$ galaxies. We find that the half light radius
of galaxies depends slightly on the color bands, consistent with the expected
distribution of star-forming regions for late type galaxies and with the known
color gradient for early type galaxies. We also show that the (inverse)
concentration index, defined by the ratio of the half light Petrosian radius to
the 90% light Petrosian radius, correlates tightly with the morphological type;
this index allows us to classify galaxies into early (E/S0) and late (spiral
and irregular) types, allowing for a 15-20% contamination from the opposite
class compared with eye-classified morphology.
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astro-ph/0504580
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Milos Milosavljevic
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What is L*?: Anatomy of the Galaxy Luminosity Function
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astro-ph
|
Using the empirical relations between the central galaxy luminosity and the
halo mass, and between the total galaxy luminosity in a halo and the halo mass,
we construct the galaxy luminosity function (LF). To the luminosity of the
central galaxy in a halo of a given mass we assign log-normal scatter with a
mean calibrated against the observations. In halos where the total galaxy
luminosity exceeds that of the central galaxy, satellite galaxies are
distributed as a power-law in luminosity. Combined with the halo mass function,
this description reproduces the observed characteristics of the galaxy LF,
including a shape consistent with the Schechter function. When all galaxies are
included, regardless of the environment or the Hubble type, the Schechter L* is
the luminosity scale above which the central galaxy luminosity-halo mass
relation flattens; L* corresponds to ~10^{13}Msun on the halo mass scale. In
surveys where central galaxies in massive clusters are neglected, either by
design or because of the cosmic variance, L* is simply the mean luminosity of
central galaxies in halos at the upper end of the selected mass range. The
smooth, exponential decay of the Schechter function toward high luminosities
reflects the intrinsic scatter in the central galaxy luminosity-halo mass
relation. In addition to the LF, the model successfully reproduces the observed
dependence of galaxy clustering bias on luminosity.
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0801.2350
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Spiga Daniele
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Estimation of X-ray scattering impact in imaging degradation for the
SIMBOL-X telescope
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astro-ph
|
The imaging performance of X-ray optics (expressed in terms of HEW,
Half-Energy-Width) can be severely affected by X-ray scattering caused by the
surface roughness of the mirrors. The impact of X-ray scattering has an
increasing relevance for increasing photon energy, and can be the dominant
problem in a hard X-ray telescope like SIMBOL-X. In this work we show how, by
means of a novel formalism, we can derive a surface roughness tolerance - in
terms of its power spectrum - from a specific HEW requirement for the SIMBOL-X
optical module.
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astro-ph/9506093
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Preprint account
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The environments of z<0.3 QSOs
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astro-ph
|
We have carried out an investigation of the galaxy environments of low
redshift ($z<0.3$) QSOs by cross-correlating the positions on the sky of
X-ray-selected QSOs/AGN identified in the {\it Einstein} Medium Sensitivity
Survey (EMSS) with those of $B_J<20.5$ galaxies in the APM galaxy catalogues.
At $<5\,$arcmin, we find a significant ($5\sigma$) galaxy excess around $z<0.3$
QSOs. The amplitude of the low redshift ($z<0.3$) QSO-galaxy angular
cross-correlation function is identical to that of the APM galaxy angular
correlation function, implying that these (predominantly radio-quiet) QSOs
inhabit environments similar to those of normal galaxies. No significant galaxy
excess was found around a `control' sample of $z>0.3$ QSOs. Coupled with
previous observations, these results imply that the environment of radio-quiet
QSOs undergoes little evolution over a wide range in redshift ($0 less than z
less than 1.5$). This is in marked contrast to the rapid increase in the
richness of the environments associated with radio-loud QSOs over the same
redshift range. The similarity between QSO-galaxy clustering and galaxy-galaxy
clustering also suggests that QSOs are unbiased with respect to galaxies and
make useful tracers of large-scale structure in the Universe.
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astro-ph/0201519
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Andrei Gruzinov
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Non-Radiative Accretion and Thermodynamics
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astro-ph
|
It has been suggested that the laws of thermodynamics are violated by what we
have called a convection-dominated accretion flow (or a 1/2-law accretion flow)
-- an accretion flow characterized by a constant outflow of energy. We show
that both the 1/2-law flow and the Bondi flow (also known as ADAF, advection
dominated accretion flow) are thermodynamically admissible.
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astro-ph/0411594
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Martino Romaniello
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The influence of chemical composition on the properties of Cepheid stars
I - Period-Luminosity relation vs iron abundance
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astro-ph
|
We have assessed the influence of the stellar iron content on the Cepheid
Period-Luminosity (PL) relation by relating the V band residuals from the
Freedman et al (2001) PL relation to [Fe/H] for 37 Galactic and Magellanic
Clouds Cepheids. The iron abundances were measured from FEROS and UVES
high-resolution and high-signal to noise optical spectra. Our data indicate
that the stars become fainter as metallicity increases, until a plateau or
turnover point is reached at about solar metallicity. Our data are incompatible
with both no dependence of the PL relation on iron abundance, and with the
linearly decreasing behavior often found in the literature (e.g. Kennicutt et
al 1998, Sakai et al 2004). On the other hand, non-linear theoretical models of
Fiorentino et al (2002) provide a fairly good description of the data.
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0802.2895
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Robert Blum
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The Ionizing Stars of the Galactic Ultra-Compact HII Region G45.45+0.06
|
astro-ph
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Using the NIFS near-infrared integral-field spectrograph behind the facility
adaptive optics module, ALTAIR, on Gemini North, we have identified several
massive O-type stars that are responsible for the ionization of the Galactic
Ultra-Compact HII region G45.45+0.06. The sources ``m'' and ``n'' from the
imaging study of Feldt et a. 1998 are classified as hot, massive O-type stars
based on their K-band spectra. Other bright point sources show red and/or
nebular spectra and one appears to have cool star features that we suggest are
due to a young, low-mass pre-main sequence component. Still two other embedded
sources (``k'' and ``o'' from Feldt et al.) exhibit CO bandhead emission that
may arise in circumstellar disks which are possibly still accreting. Finally,
nebular lines previously identified only in higher excitation planetary nebulae
and associated with KrIII and SeIV ions are detected in G45.45+0.06.
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astro-ph/9811338
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Alexei A. Pamyatnykh
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The Delta Scuti star FG Vir. IV. Mode identifications and pulsation
modelling
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astro-ph
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This paper examines the mode identification and presents pulsation models for
FG Vir, for which 24 frequencies have been detected. Histograms of the
frequency spacings show peaks which are identified with adjacent radial orders
and rotational splitting.
Pulsational $\ell$ values are deduced for eight modes by comparing the
observed photometric phase lags between $v$ and $y$ variations with calculated
values. The dominant pulsation mode at 12.72 c/d can be identified with $\ell$
= 1, while the 12.15 c/d mode is the radial fundamental. These results are in
agreement with identifications published by Viskum et al. (1998).
Based on the observational mode identifications and the Hipparcos distance,
new models were computed with the constraint that the mode at 12.15 c/d is the
radial fundamental mode. It is shown that with standard opacities, models in
the appropriate T_eff, log L and log g ranges cannot reproduce the
identification in the literature of 23.40 c/d as the third radial overtone.
However, we show that observationally an $\ell$ = 1 (rather than radial)
identification is equally probable.
A large number of pulsation models were computed for FG Vir. A comparison
between the observed frequencies and mode identifications and pulsation models
leads to a mean density of <rho>/<rho_sun> = 0.156 +- 0.002 depending on the
opacity and chemical composition choice and on the possible overshooting from
the convective core. The models also correctly predict the observed region of
instability between 9 and 34 c/d.
The effect of rotational coupling on the pulsation frequencies is estimated.
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astro-ph/0207214
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Unal Ertan
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On the Outbursts of Soft X-ray Transients
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astro-ph
|
We suggest a new scenario to explain the outburst light curves of black hole
soft X-ray transients together with the secondary maximum and the bump seen on
their decay phases. Our explanations are based on the disk instability models
considering the effect of X-ray irradiation. The scenario is consistent with
the observed X-ray delays by a few days with respect to the optical rise for
both the main outburst and the later maxima. We test our scenario by
numerically solving the disk diffusion equation. The obtained model curve fits
well to the observed X-ray outburst photon flux curve of the black hole soft
X-ray transient GS/GRS 1124-68, a typical representative of the four BH SXTs
with very similar light curves.
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astro-ph/0005028
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Donald Q. Lamb
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Implications of Recent Observational Discoveries for the Nature and
Origin of Gamma-Ray Bursts
|
astro-ph
|
The discoveries that GRBs have X-ray, optical and radio afterglows have
connected the study of GRBs to the rest of astronomy, and revolutionized the
field. In this review, I discuss the implications that the observation of these
afterglows have for burst energies and luminosities, and for models of the
bursts and their afterglows. I describe recent evidence linking the long,
softer, smoother GRBs detected by BeppoSAX and core collapse supernovae.
Finally, I summarize recent work showing that, if these GRBs are due to the
collapse of massive stars, they may provide a powerful probe of the very high
redshift universe.
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astro-ph/9902231
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Harry Teplitz
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Near-Infrared Observations of the Environments of Radio Quiet QSOs at z
>~ 1
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astro-ph
|
We present the results of an infrared survey of QSO fields at z=0.95, 0.995
and 1.5. Each z<1 field was imaged to typical continuum limits of J=20.5,
Kprime=19 (5 sigma), and line fluxes of 1.3E10{-16}ergs/cm^2/s (1 sigma)in a 1%
interference filter. 16 fields were chosen with z~0.95 targets, 14 with z~0.995
and 6 with z~1.5. A total area of 0.05 square degrees was surveyed, and two
emission-line objects were found. We present the infrared and optical
photometry of these objects. Optical spectroscopy has confirmed the redshift of
one object (at z=0.989) and is consistent with the other object having a
similar redshift. We discuss the density of such objects across a range of
redshifts from this survey and others in the literature. We also present
number-magnitude counts for galaxies in the fields of radio quiet QSOs,
supporting the interpretation that they exist in lower density environments
than their radio loud counterparts. The J-band number counts are among the
first to be published in the J=16--20.
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astro-ph/0501355
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Yeshe Fenner
|
Stellar abundance gradients in galactic disks. I. Method and spectral
line gradients
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astro-ph
|
We describe the technique of absorption line imaging of galaxy disks using
the Taurus Tunable Filter on the Anglo-Australian Telescope and demonstrate its
sensitivity to the behaviour of spectral features associated with Mg and Fe.
Radial profiles of Mg2 and Fe5270 line-strengths are presented for a sample of
eight face-on spiral galaxies spanning a range of Hubble types. Signatures of
phenomena including merger-induced star formation, HII rings and galactic bars
are also reported. This study demonstrates the capacity of tunable filters to
measure Mg and Fe line-strengths across the face of spiral galaxies, which can
ultimately reveal clues about the star formation history and chemical
evolution.
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0809.0359
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Helmut Wiesemeyer
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Precessing planetary magnetospheres in SiO stars ? First detection of
quasi-periodic polarization fluctuations in R Leo and V Cam
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astro-ph
|
The origin of magnetism around AGB stars is uncertain. If these stars drive a
dynamo, it cannot be sufficient to generate a strong global field, otherwise
the observed X-ray luminosities would be higher. Other explanations for the
circular polarization of SiO masers in the atmospheres are needed. The
interaction of the AGB wind with previously ejected matter and with planets
bears complex magneto-hydrodynamic phenomena on a short time scale, such that
strong magnetic fields can be maintained locally. Here we provide observational
evidence for the corresponding magnetic fluctuations. We trace magnetic
activity with the circular polarization of the v=1, J=2-1 SiO masers, using a
correlation polarimeter. In V Cam and R Leo, we find evidence of
pseudo-periodic fluctuations of the circular polarization on a timescale of a
few hours, from which we infer magnetic fluctuations of ~1G. The phenomenon is
rare and restricted to a narrow range of velocities. It seems to be associated
with planetary wake flows suggested by VLBI maps. While scenarios involving
magnetic activity in the extended stellar atmosphere have problems to explain
all observed features, precessing Jovian magnetospheres predict all of them.
For the case of R Leo, we constrain the orbit of the planet (estimated period
5.2 years) and estimate a stellar mass of 0.7 solar masses.
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astro-ph/0701006
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Robert L. Oldershaw
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Hadrons As Kerr-Newman Black Holes
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astro-ph
|
The scale invariance of the source-free Einstein field equations suggests
that one might be able to model hadrons as "strong gravity" black holes, if one
uses an appropriate rescaling of units or a revised gravitational coupling
factor. The inner consistency of this hypothesis is tested by retrodicting a
close approximation to the mass of the proton from an equation that relates the
angular momentum and mass of a Kerr black hole. More accurate mass and radius
values for the proton are then retrodicted using the geometrodynamics form of
the full Kerr-Newman solution of the Einstein-Maxwell equations. The radius of
an alpha particle is calculated as an additional retrodictive test. In a third
retrodictive test of the "strong gravity" hypothesis, the subatomic particle
mass spectrum in the 100 MeV to 7,000 MeV range is retrodicted to a first
approximation using the Kerr solution of General Relativity. The particle
masses appear to form a restricted set of quantized values of the Kerr
solution: n^1/2 M, where values of n are a set of discrete integers and M is
the revised Planck mass. The accuracy of the 27 retrodicted masses averages
98.4%. Finally, the new atomic scale gravitational coupling constant suggests a
radical revision of the assumptions governing the Planck scale, and leads to a
natural explanation for the fine structure constant.
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astro-ph/9607075
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Paolo Goldoni
|
Isolated neutron stars, their $\gamma$-ray efficiencies and EGRET
observations
|
astro-ph
|
We examine a sample of detected and undetected Isolated Neutron Stars (INSs),
selected on the basis of their energy loss and distance, in order to maximize
their detection probability. Since the sample we consider encompasses more
upper limits than detections, we make use, for the first time in this field of
astronomy, of survival analysis procedures through the ASURV software package
(Feigelson \& Nelson 1985, Isobe et al. 1986). We show that these techniques
lead to a better understanding of the physical processes at work in high-energy
emission from INSs. In particular, the recent detection of PSR 1951+32 and
upper limits from pulsars pointed but not detected by EGRET show that the
$\gamma $-ray efficiency of ISN is not correlated to any simple pulsar
parameter.
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astro-ph/0508195
|
William Harris
|
Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal
Metallicity Distributions and the Nature of the High-Luminosity Clusters
|
astro-ph
|
We present new (B,I) photometry for the globular cluster systems in eight
Brightest Cluster Galaxies (BCGs), obtained with the ACS/WFC camera on the
Hubble Space Telescope. In the very rich cluster systems that reside within
these giant galaxies, we find that all have strongly bimodal color
distributions All the BCGs show population gradients, with much higher relative
numbers of red clusters within 5 kpc of their centers, consistent with their
having formed at later times than the blue, metal-poor population. A striking
new feature of the color distributions emerging from our data is that for the
brightest clusters (M_I < -10.5) the color distribution becomes broad and less
obviously bimodal. we suggest that it may be a characteristic of many BCGs.
Furthermore, the blue (metal-poor) clusters become progressively redder with
increasing luminosity, following a mass/metallicity scaling relation Z ~
M^0.55. We argue that these GCS characteristics are consistent with a
hierarchical-merging formation picture in which the metal-poor clusters formed
in protogalactic clouds or dense starburst complexes with gas masses in the
range 10^7 - 10^10 M_Sun, but where the more massive clusters on average formed
in bigger clouds with deeper potential wells where more pre-enrichment could
occur.
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astro-ph/0211074
|
Seppo Laine
|
Hubble Space Telescope Imaging of Brightest Cluster Galaxies
|
astro-ph
|
We used the HST WFPC2 to obtain I-band images of the centers of 81 brightest
cluster galaxies (BCGs), drawn from a volume-limited sample of nearby BCGs. The
images show a rich variety of morphological features, including multiple or
double nuclei, dust, stellar disks, point source nuclei, and central surface
brightness depressions. High resolution surface brightness profiles could be
inferred for 60 galaxies. Of those, 88% have well-resolved cores. Twelve
percent of the BCG sample lacks a well-resolved core; all but one of these BCGs
have ``power-law'' profiles. Some of these galaxies have higher luminosities
than any power-law galaxy identified by Faber et al. (1997), and have physical
upper limits on the break radius well below the values observed for core
galaxies of the same luminosity. These results support the idea that the
central structure of early-type galaxies is bimodal in its physical properties,
but also suggest that there exist high luminosity galaxies with power-law
profiles (or unusually small cores). The BCGs in the latter category tend to
fall at the low end of the BCG luminosity function and tend to have low values
of the quantity alpha (the logarithmic slope of the metric luminosity as a
function of radius, at 10 kpc). Since theoretical calculations have shown that
the luminosities and alpha values of BCGs grow with time as a result of
accretion, this suggests a scenario in which elliptical galaxies evolve from
power-law profiles to core profiles through accretion and merging. This is
consistent with theoretical scenarios that invoke the formation of massive
black hole binaries during merger events (Abridged).
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No Label
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astro-ph/9905222
|
Wendy L. Freedman
|
Determination of Cosmological Parameters
|
astro-ph
|
Rapid progress has been made recently toward the measurement of cosmological
parameters. Still, there are areas remaining where future progress will be
relatively slow and difficult, and where further attention is needed. In this
review, the status of measurements of the matter density, the vacuum energy
density or cosmological constant, the Hubble constant, and ages of the oldest
measured objects are summarized. Many recent, independent dynamical
measurements are yielding a low value for the matter density of about 1/3 the
critical density. New evidence from type Ia supernovae suggests that the vacuum
energy density may be non-zero. Many recent Hubble constant measurements appear
to be converging in the range of 65-75 km/sec/Mpc. Eliminating systematic
errors lies at the heart of accurate measurements for all of these parameters;
as a result, a wide range of cosmological parameter space is currently still
open. Fortunately, the prospects for accurately measuring cosmological
parameters continue to increase.
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No Label
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No Label
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astro-ph/0406223
|
Thomas Roellig
|
The Spitzer Space Telescope Mission
|
astro-ph
|
The Spitzer Space Telescope, NASA's Great Observatory for infrared astronomy,
was launched 2003 August 25 and is returning excellent scientific data from its
Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity
achievable with a cryogenic telescope in space with the great imaging and
spectroscopic power of modern detector arrays to provide the user community
with huge gains in capability for exploration of the cosmos in the infrared.
The observatory systems are largely performing as expected and the projected
cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit
scientific, technical and operational performance of Spitzer. Subsequent papers
in this special issue describe the Spitzer instruments in detail and highlight
many of the exciting scientific results obtained during the first six months of
the Spitzer mission.
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No Label
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No Label
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astro-ph/0503654
|
Walter Nowotny
|
Synthetic Line Profiles for Pulsating Red Giants
|
astro-ph
|
Pulsation influences atmospheric structures of variable AGB stars (Miras)
considerably. Spectral lines of the CO dv=3 vibration-rotation bands (at
1.6mue) therefore have a very characteristic appearance in time series of
high-resolution spectra. Coupled to the light cycle they can be observed blue-
or red-shifted, for some phases even line doubling is found. This is being
explained by radial pulsations and shock fronts emerging in the atmospheres.
Based on dynamic model atmospheres synthetic CO line profiles were calculated
consistently, reproducing this scenario qualitatively.
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No Label
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No Label
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astro-ph/0110231
|
Robert Nichol
|
Large-Scale Structure Studies with Clusters of Galaxies
|
astro-ph
|
I present here a review of Large-Scale Structure (LSS) studies using clusters
of galaxies. First, I re-evaluate the `pros' and `cons' of using clusters for
such studies, especially in this era of large galaxy redshift surveys.
Secondly, I provide an historical review of the Cluster Correlation Function
and show that the latest measurements of xi_cc from Abell and X-ray catalogs
are in excellent agreement. Thirdly, I review the latest measurements of the
power spectrum of clusters which provide strong constraints on the cosmological
parameters e.g. Omega_m and models of structure formation. Moreover, I
highlight the recent discovery of ``Baryon Wiggles'' in the local cluster p(k)
which is in perfect agreement with the recent CMB data. Lastly, I examine
recent advances in the measurement of the X-ray Cluster Luminosity Function and
emphasize the importance of accurately determining the selection function of
future cluster surveys.
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No Label
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No Label
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astro-ph/9708087
|
Q. Daniel Wang
|
ROSAT and ASCA observations of the Crab-Like Supernova Remnant N157B in
the Large Magellanic Cloud
|
astro-ph
|
We report the results of ROSAT and ASCA X-ray observations of the supernova
remnant N157B (or 30 Dor B, SNR 0539-69.1) in the Large Magellanic Cloud. For
comparison, we also briefly describe the results on SNR 0540-69.3, the only
confirmed Crab-like remnant in the Cloud. The X-ray emission from N157B can be
decomposed into a bright comet-shaped feature, superimposed on a diffuse
emission region of a dimension $\sim 20$ pc. The flat and nearly featureless
spectrum of the remnant is distinctly different from those of young shell-like
remnants, suggesting a predominantly Crab-like nature of N157B. Characterized
by a power law with an energy slope $\sim 1.5$, the spectrum of N157B above
$\sim 2$ keV is, however, considerably steeper than that of SNR 0540-69.3,
which has a slope of $\sim 1.0$. At lower energies, the spectrum of N157B
presents marginal evidence for emission lines, which if real most likely arise
in hot gas of the diffuse emission region. The hot gas has a characteristic
thermal temperature of 0.4-0.7 keV. No significant periodic signal is detected
from N157B in the period range of $3 \times 10^{-3}-2000$ s. The pulsed
fraction is $\lesssim 9%$ (99% confidence) in the $2-7$ keV range. We discuss
the nature of the individual X-ray components. In particular, we suggest that
the synchrotron radiation of relativistic particles from a fast-moving ($\sim
10^3 km s^{-1}$) pulsar explains the size, morphology, spectrum, and energetics
of the comet-shaped X-ray feature. We infer the age of the remnant as $\sim 5
\times 10^3$ yrs. The lack of radio polarization of the remnant may be due to
Faraday dispersion by foreground \ion{H}{2} gas.
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No Label
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No Label
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astro-ph/9706162
|
David Chernoff
|
Neutron Star Population Dynamics.I: Millisecond Pulsars
|
astro-ph
|
We study the field millisecond pulsar (MSP) population to infer its intrinsic
distribution in spin period and luminosity and to determine its spatial
distribution within the Galaxy. Our likelihood analysis on data from extant
surveys (22 pulsars with periods <20 ms) accounts for important selection
effects. We infer a minimum period cutoff P(min) > 0.65 ms (99% confidence), a
period distribution proportional to P^{-2.0 +- 0.33} and a pseudo-luminosity
distribution proportional to L_p^{-2.0 +- 0.2} (where L_p = flux density times
distance^2, for L_p >= 1.1 mJy kpc^2). We find a vertical scale height
0.65{+0.16,-0.12} kpc. We use our results to estimate the total number and
birthrate of MSPs in the disk of the Galaxy. We limit the density contribution
of a diffuse halo-like component to <1% of the midplane value. The MSP velocity
dispersion is smaller that that of young, long-period pulsars by about a factor
of 5. Our best estimate of the 1D velocity kick that is unique to MSP evolution
is approximately 40 km s^-1. We discuss the evolutionary relationship of MSPs
and low-mass X-ray binaries and prospects for future searches for MSPs.
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astro-ph/0310076
|
Hugo Schwarz E.
|
Kinematics and Proper Motion of the Ansae in NGC7009
|
astro-ph
|
We have measured the expansion velocities and proper motion of the ansae in
NGC7009 using high dispersion echelle spectra and archive narrow band HST
images. Assuming that the ansae are moving at equal and opposite velocities
from the central star we obtain an average system radial velocity of -54 +-2
km/s, the eastern ansa approaching and the western ansa receding at Vr = 5.5
+-1 km/s relative to this value. Only the proper motion of the eastern ansa
could be measured, leading to 2.8 +- 0.8 arcsec/century, or Vt=130 +- 40.d
km/s, where d is the distance to the nebula in kpc. Additionally, the electron
temperature and density for each ansa was measured using line intensity ratios.
The results are Te = 9000 K and ne = 2000 /cm3 for both ansae within the
errors.
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No Label
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No Label
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astro-ph/0312426
|
Bart Willems
|
Constraints on the Formation of PSR J0737-3039: the most probable
isotropic kick magnitude
|
astro-ph
|
A strongly relativistic binary pulsar has been recently discovered with the
64m Parkes telescope (Burgay et al. 2003). Here we use the measured properties
of this binary (masses and orbital characteristics as well as age estimates),
and we derive the complete set of constraints imposed on the physical
properties of the binary pulsar progenitor right before the second supernova
explosion. We find that: (i) according to our current understanding of
neutron-star formation, the helium-rich progenitor of the second neutron star
is most likely overflowing its Roche lobe; (ii) the neutron-star kick magnitude
is constrained in the range 60-1560 km/s, with the most probable value being
equal to 150 km/s. While the first conclusion is in agreement with Dewi & van
den Heuvel (2003), our upper limit on the kick magnitude is significantly
larger than that derived by these authors. We find that the difference arises
because Dewi & van den Heuvel (2003) inadvertently neglected to consider kicks
directed out of the pre-supernova orbital plane.
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astro-ph/9904155
|
Karen Leighly
|
An RXTE Observation of NGC 6300: a new bright Compton reflection
Dominated Seyfert 2 Galaxy
|
astro-ph
|
Scanning and pointed RXTE observations of the nearby Seyfert 2 galaxy NGC
6300 reveal that it is a source of hard X-ray continuum and large equivalent
width Fe K$\alpha$ emission. These properties are characteristic of
Compton-reflection dominated Seyfert 2 galaxies. The continuum can be modeled
as Compton-reflection; subsolar iron abundance is required and a high
inclination preferred. However, the poor energy resolution of RXTE means that
this description is not unique, and the continuum can also be modeled using a
``dual absorber'', i.e. a sum of absorbed power laws. Observations with higher
energy resolution detectors will cleanly discriminate between these two models.
Optical observations support the Compton-reflection dominated interpretation as
$L_X/L_{[OIII]}$ is low. NGC 6300 is notable because with $F_{2-10}\approx 6.4
\times 10^{-12} ergs cm^{-2} s^{-1}$, it is the second brightest such object
known.
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No Label
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astro-ph/0303640
|
Leslie Looney
|
Envelope Emission in Young Stellar Systems: A Sub-Arcsecond Survey of
Circumstellar Structure
|
astro-ph
|
We present modeling results for six of the eleven deeply embedded systems
from our sub-arcsecond 2.7 mm wavelength continuum interferometric survey. The
modeling, performed in the uv plane, assumes dust properties, allows for a
power-law density profile, uses a self-consistent, luminosity conserving
temperature profile, and has an embedded point source to represent a
circumstellar disk. Even though we have the highest spatial resolution to date
at these wavelengths, only the highest signal-to-noise systems can adequately
constrain the simple self-similar collapse models. Of the six sources modeled,
all six were fit with a density power-law index of 2.0; however, in half of the
systems, those with the highest signal-to-noise, a density power-law index of
1.5 can be rejected at the 95% confidence level. Further, we modeled the
systems using the pure Larson-Penston (LP) and Shu solutions with only age and
sound speed as parameters. Overall, the LP solution provides a better fit to
the data, both in likelihood and providing the observed luminosity, but the age
of the systems required by the fits are surprising low (1000-2000 yrs). We
suggest that either there is some overall time scaling of the self-similar
solutions that invalidate the age estimates, or more likely we are at the limit
of the usefulness of these models. With our observations we have begun to reach
the stage where models need to incorporate more of the fundamental physics of
the collapse process, probably including magnetic fields and/or turbulence. In
addition to constraining collapse solutions, our modeling allows the separation
of large-scale emission from compact emission, enabling the probing of the
circumstellar disk component embedded within the protostellar envelope.
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No Label
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No Label
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astro-ph/9908286
|
Padeli P. Papadopoulos
|
CO(4-3) and dust emission in two powerful high-z radio galaxies, and CO
lines at high redshifts
|
astro-ph
|
We report the detection of sub-mm emission from dust at 850 microns and of
the 12CO J=4-3 line in the two distant powerful radio galaxies 4C 60.07
(z=3.79) and 6C 1909+722 (z=3.53). In the case of 4C 60.07 the dust emission is
also detected at 1.25 mm. The estimated molecular gas masses are large, of the
order of ~(0.5-1)x10^{11} Solar. The large FIR luminosities (L_fir ~ 10^{13}
Solar) suggest that we are witnessing two major starburst phenomena, while the
observed large velocity widths (FWHM > 500 km/sec) are characteristic of
mergers. In the case of 4C 60.07 the CO emission extends over ~30 kpc and spans
a velocity range of >1000 km/sec. It consists of two distinct features with
FWHM of >= 550 km/sec and ~150 km/sec and line centers separated by >=700
km/sec The least massive of these components is probably very gas-rich with
potentially >=60% of its dynamical mass in the form of molecular gas. The
extraordinary morphology of the CO emission in this object suggests that it is
not just a scaled-up version of a local Ultra Luminous Infrared Galaxy, and it
may be a formative stage of the elliptical host of the residing radio-loud AGN.
Finally we briefly explore the effects of the wide range of gas excitation
conditions expected for starburst environments on the luminosity of high-J CO
lines. We conclude that in unlensed objects, CO (J+1-->J), J+1>3 lines can be
significantly weak with respect to CO J=1-0 and this can hinder their detection
even in the presence of substantial molecular gas masses.
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No Label
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No Label
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0802.1746
|
Nathan Smith
|
Eta Carinae and Nebulae Around Massive Stars: Similarities to Planetary
Nebulae?
|
astro-ph
|
I discuss some observational properties of aspherical nebulae around massive
stars, and conclusions inferred for how they may have formed. Whether or not
these ideas are applicable to the shaping of planetary nebulae is uncertain,
but the observed similarities between some PNe and bipolar nebulae around
massive stars is compelling. In the well-observed case of Eta Carinae, several
lines of observational evidence point to a scenario where the shape of its
bipolar nebula resulted from an intrinsically bipolar explosive ejection event
rather than an interacting winds scenario occurring after ejection from teh
star. A similar conclusion has been inferred for some planetary nebulae. I also
briefly mention bipolar nebulae around some other massive stars, such as the
progenitor of SN 1987A and related blue supergiants.
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No Label
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No Label
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astro-ph/9906182
|
Virgo software development
|
Spectral Analysis of Stellar Light Curves by Means of Neural Networks
|
astro-ph
|
Periodicity analysis of unevenly collected data is a relevant issue in
several scientific fields. In astrophysics, for example, we have to find the
fundamental period of light or radial velocity curves which are unevenly
sampled observations of stars. Classical spectral analysis methods are
unsatisfactory to solve the problem. In this paper we present a neural network
based estimator system which performs well the frequency extraction in unevenly
sampled signals. It uses an unsupervised Hebbian nonlinear neural algorithm to
extract, from the interpolated signal, the principal components which, in turn,
are used by the MUSIC frequency estimator algorithm to extract the frequencies.
The neural network is tolerant to noise and works well also with few points in
the sequence. We benchmark the system on synthetic and real signals with the
Periodogram and with the Cramer-Rao lower bound.
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No Label
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No Label
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astro-ph/0406362
|
Agnieszka Janiuk
|
Evolution of a neutrino-cooled disc in Gamma-Ray Bursts
|
astro-ph
|
Rapid, hyper-Eddington accretion is likely to power the central engines of
gamma-ray bursts (GRBs). In the extreme conditions of densities and
temperatures the accreting torus is cooled by neutrino emission rather than by
radiation. Another important cooling mechanism is the advection of energy into
the central black hole. We compute the time evolution of a neutrino-dominated
disc that proceeds during the burst and investigate the changes in its density
and temperature. The discrimination between short and long bursts is made on
the basis of the different rates of material inflow to the outer parts of the
disc, thus favoring the binary merger scenario for the short GRBs and the
collapsar scenario for the long ones. Within the context of the collapsar
model, we also study the evolution of the photon luminosity of the remnant disc
up to times of the order of 1 day, and discuss its implications for the
production of emission lines in GRB spectra.
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No Label
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No Label
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astro-ph/0104125
|
Shai Kaspi
|
High-Resolution X-ray and Ultraviolet Spectroscopy of the Complex
Intrinsic Absorption in NGC 4051 with Chandra and HST
|
astro-ph
|
We present the results from simultaneous observations of the Narrow-Line
Seyfert 1 galaxy NGC 4051 with the Chandra High Energy Transmission Grating
Spectrometer and the HST Space Telescope Imaging Spectrograph. The X-ray
grating spectrum reveals absorption and emission lines from hydrogen-like and
helium-like ions of O, Ne, Mg and Si. We resolve two distinct X-ray absorption
systems: a high-velocity blueshifted system at -2340+/-130 km/s and a
low-velocity blueshifted system at -600+/-130 km/s. In the UV spectrum we
detect strong absorption, mainly from C IV, N V and Si IV, that is resolved
into as many as nine different intrinsic absorption systems with velocities
between -650 km/s and 30 km/s. Although the low-velocity X-ray absorption is
consistent in velocity with many of the UV absorption systems, the
high-velocity X-ray absorption seems to have no UV counterpart. In addition to
the absorption and emission lines, we also observe rapid X-ray variability and
a state of low X-ray flux during the last ~15 ks of the observation. NGC 4051
has a soft X-ray excess which we fit in both the high and low X-ray flux
states. The high-resolution X-ray spectrum directly reveals that the soft
excess is not composed of narrow emission lines and that it has significant
spectral curvature. A power-law model fails to fit it, while a blackbody
produces a nearly acceptable fit. We compare the observed spectral variability
with the results of previous studies of NGC 4051.
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No Label
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astro-ph/0007046
|
Garcia
|
Riemannian collineations in General Relativity and in Einstein-Cartan
cosmology
|
astro-ph
|
Riemannian vectorial collineations along with current Killing conservation
are shown to lead to tensorial collineations for the energy-stress tensor in
general relativity and in Einstein-Cartan Weyssenhoff fluid cosmology.
|
No Label
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No Label
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astro-ph/0601174
|
Shogo Nishiyama
|
Interstellar Extinction Law in the J, H, and Ks Bands toward the
Galactic Center
|
astro-ph
|
We have determined the ratios of total to selective extinction in the
near-infrared bands (J, H, Ks) toward the Galactic center from the observations
of the region |l| < 2.0deg and 0.5deg < |b| < 1.0deg with the IRSF telescope
and the SIRIUS camera. Using the positions of red clump stars in
color-magnitude diagrams as a tracer of the extinction and reddening, we
determine the average of the ratios of total to selective extinction to be
A(Ks)/E(H-Ks) = 1.44+-0.01, A(Ks)/E(J-Ks) = 0.494+-0.006, and A(H)/E(J-H) =
1.42+-0.02, which are significantly smaller than those obtained in previous
studies. From these ratios, we estimate that A(J) : A(H) : A(Ks) = 1 :
0.573+-0.009 : 0.331+-0.004 and E(J-H)/E(H-Ks) = 1.72+-0.04, and we find that
the power law A(lambda) \propto lambda^{-1.99+-0.02} is a good approximation
over these wavelengths. Moreover, we find a small variation in A(Ks)/E(H-Ks)
across our survey. This suggests that the infrared extinction law changes from
one line of sight to another, and the so-called ``universality'' does not
necessarily hold in the infrared wavelengths.
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No Label
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No Label
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0709.3754
|
Garrelt Mellema
|
Numerical simulations of type III planetary migration: II. Inward
migration of massive planets
|
astro-ph
|
We present a numerical study of rapid, so called type III migration for
Jupitersized planets embedded in a protoplanetary disc. We limit ourselves to
the case of inward migration, and study in detail its evolution and physics,
concentrating on the structure of the corotation and circumplanetary regions,
and processes for stopping migration. We also consider the dependence of the
migration behaviour on several key parameters. We perform this study using the
results of global, two-dimensional hydrodynamical simulations with adaptive
mesh refinement. The initial conditions are chosen to satisfy the condition for
rapid inward migration. We find that type III migration can be divided into two
regimes, fast and slow. The structure of the coorbital region, mass
accumulation rate, and migration behaviour differ between these two regimes.
All our simulations show a transition from the fast to the slow regime, ending
type III migration well before reaching the star. The stopping radius is found
to be larger for more massive planets and less massive discs. A sharp density
drop is also found to be an efficient stopping mechanism. In the fast migration
limit the migration rate and induced eccentricity are lower for less massive
discs, but almost do not depend on planet mass. Eccentricity is damped on the
migration time scale.
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No Label
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No Label
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astro-ph/9807055
| null |
Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant
Planet Atmospheres
|
astro-ph
|
We calculate detailed chemical abundance profiles for a variety of brown
dwarf and extrasolar giant planet atmosphere models, focusing in particular on
Gliese 229B, and derive the systematics of the changes in the dominant
reservoirs of the major elements with altitude and temperature. We assume an
Anders and Grevesse (1989) solar composition of 27 chemical elements and track
330 gas--phase species, including the monatomic forms of the elements, as well
as about 120 condensates. We address the issue of the formation and composition
of clouds in the cool atmospheres of substellar objects and explore the rain
out and depletion of refractories. We conclude that the opacity of clouds of
low--temperature ($\le$900 K), small--radius condensibles (specific chlorides
and sulfides), may be responsible for the steep spectrum of Gliese 229B
observed in the near infrared below 1 \mic. Furthermore, we assemble a
temperature sequence of chemical transitions in substellar atmospheres that may
be used to anchor and define a sequence of spectral types for substellar
objects with T$_{eff}$s from $\sim$2200 K to $\sim$100 K.
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astro-ph/9501015
|
Helene Di Nella
|
Structure on large scale in the Universe up to the distance of 200 Mpc
|
astro-ph
|
The distribution of galaxies up to a distance of 200 Mpc (650 million
light-years) is flat and shows a structure like a shell roughly centered on the
Local Supercluster (Virgo cluster). This result clearly confirms the existence
of the hypergalactic large scale structure noted in 1988. This is presently the
largest structure ever seen.
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No Label
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No Label
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astro-ph/0409614
|
Enno Middelberg
|
VLBI detections of a source weaker than 100 mJy at 86 GHz
|
astro-ph
|
We use a new phase-calibration strategy to calibrate the phase of 86 GHz VLBI
observations of the FR I radio galaxy NGC 4261. Instead of switching between a
calibrator source and the target source, the target was observed while rapidly
switching between the target frequency and a lower reference frequency.
Self-calibration at the reference frequency yielded phase corrections which
were multiplied with the frequency ratio and applied to the target frequency
visibilities. The resulting detection of NGC 4261 is, to our knowledge, the
first of NGC 4261 with 86 GHz VLBI, and it is also the weakest source so far
detected with VLBI at that frequency.
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No Label
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No Label
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astro-ph/0301200
|
Markus Landgraf
|
Comparison of EISCAT Radar Data on Space Debris with Model Prediction by
the MASTER Model of ESA
|
astro-ph
|
In the effort to obtain low cost routine space debris observations in low
Earth orbit, ESA plans to utilise the radar facilities of the European
Incoherent Scatter Scientific Association. First demonstration measurements
were performed from 11 to 23 February 2001. In total $16{\rm hours}$ of radar
signals were collected. Here we compare these initial measurements with the
predictions of the ESA MASTER/PROOF'99 model in order to assess the sensitivity
as well as the reliability of the data. We find that while the determination of
object size needs to be reviewed, the altitude distribution provides a good fit
to the model prediction. The absolute number of objects detected in the various
altitude bins indicates that the coherent integration method indeed increases
the detection sensitivity when compared to incoherent integration. In the data
presented here integration times from 0.1 to $0.3{\rm s}$ were used. As
expected, orbit information cannot be obtained from the measurements if they
are linked to ionospheric measurements as planned. In addition routine space
debris observations provide also useful information for the validation of
large-object catalogues.
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No Label
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No Label
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astro-ph/0308395
|
Osmi Vilhu
|
First INTEGRAL observations of Cygnus X-3
|
astro-ph
|
We present the first INTEGRAL results on Cyg X-3 from the PV phase
observations of the Cygnus region. The source was clearly detected by the
JEM-X, ISGRI and SPI detectors. The INTEGRAL observations were supported by
simultaneous pointed RXTE observations. Their lightcurves folded over the 4.8
hour binary period are compatible with the mean RXTE/ASM and CGRO/BATSE light
curves. We fit our broad band X-ray/Gamma-ray spectra with a physical model,
which represents the first such published model for Cyg X-3. The main physical
processes in the source are thermal Comptonization and Compton reflection with
parameters similar to those found for black-hole binaries at high Eddington
rates.
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No Label
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No Label
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astro-ph/9605153
| null |
Solar opacity, neutrino signals and helioseismology
|
astro-ph
|
In connection with the recent suggestion by Tsytovich et al. that opacity in
the solar core could be overestimated, we consider the following questions: i)
What would a 10\% opacity reduction imply for the solar neutrino puzzle? ii) Is
there any hope of solving the solar neutrino puzzle by changing opacity? iii)
Is a 10\% opacity reduction testable with helioseismological data?
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No Label
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No Label
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astro-ph/0202102
|
Mathew James Page
|
Submillimeter Evidence for the Coeval Growth of Massive Black Holes and
Galaxy Bulges
|
astro-ph
|
The correlation, found in nearby galaxies, between black hole mass and
stellar bulge mass implies that the formation of these two components must be
related. Here we report submillimeter photometry of eight x--ray absorbed
active galactic nuclei which have luminosities and redshifts characteristic of
the sources that produce the bulk of the accretion luminosity in the universe.
The four sources with the highest redshifts are detected at 850 microns, with
flux densities between 5.9 and 10.1 milliJanskies, and hence are ultraluminous
infrared galaxies. Interpreting the submillimeter flux as emission from dust
heated by starbursts, these results suggest that the majority of stars in
spheroids were formed at the same time as their central black holes built up
most of their mass by accretion, accounting for the observed demography of
massive black holes in the local universe. The skewed rate of submillimeter
detection with redshift is consistent with a high redshift epoch of star
formation in radio quiet active galactic nuclei, similar to that seen in radio
galaxies.
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