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astro-ph/0008391
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Sergio Molinari
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A Shock-Induced PDR in the HH 80/81 Flow. Far Infrared Spectroscopy
|
astro-ph
|
The two spectrometers on board the Infrared Space Observatory were used to
observe the Herbig-Haro objects HH 80, 81 and 80N, as well as their candidate
exciting source IRAS 18162-2048. The fine structure lines of [OI]63um,
[OI]145um and [CII]158um are detected everywhere, while [NII]122um and
[OIII]88um are only detected toward the HH objects; line ratios confirm for the
first time the collisionally excited HH nature of HH 80N. No molecular line is
detected in any of the observed positions. We use a full shock code to diagnose
shock velocities vs~100 km/s toward the HH objects, as expected from the
optical spectroscopy. Since proper motions suggest velocities in excess of 600
km/s, the HH objects probably represent the interface between two flow
components with velocity differing by ~vs. Aside from the flow exciting source,
the [CII]158um line is everywhere brighter than the [OI]63um line, indicating
the presence of a Photo-Dissociation Region (PDR) all along the flow. Continuum
emission from the HH objects and from other positions along the flow is only
detected longword of ~50 micron, and its proportionality to the [CII]158um line
flux suggests it is PDR in origin. We propose that the FUV continuum irradiated
by the HH objects and the jet is responsible for the generation of a PDR at the
walls of the flow cavity. We develop a very simple model which strengthens the
plausibility of this hypothesis.
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No Label
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astro-ph/0208522
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Stefan Kimeswenger
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The peculiar variable V838 Mon
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astro-ph
|
V838 Mon underwent, after a first nova-like outburst in January and a usual
decline, a second outburst after one month, and a third weak one again a month
later. Moreover a very small increase of the temperature at the beginning of
April gives us a hint on a physical process with a period of one month. We
obtained a BVRIc time sequence and modelled the photometric behaviour of the
object. This leads us to the conclusion that the interstellar foreground
extinction has to be 0.6 < E(B-V) < 0.8 and that the quasi-photosphere had
persistently unusually low temperatures for nova-like systems. The photometry
was used to follow the dramatic changes of the expansion. While the appearing
10 micron excess can be well described by the heating of material ejected
during this event, the IRAS emission near the location of the progenitor,
originates most likely from dust, which were formed during the previous
evolution of the object. Assuming that the light echoes are coming from
circumstellar material, the distance is 640 to 680 pc - smaller than the 790 pc
given in Munari et al. (2002). In our opinion V838 Mon and V4332 Sgr are
manifestations of a new class of eruptive variables. We do not count M31 RV to
this class.
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astro-ph/0508538
|
Xinwu Cao
|
Constraints on radiatively inefficient accretion history of active
galactic nuclei from hard cosmological X-ray background
|
astro-ph
|
The transition of a standard thin disk to a radiatively inefficient accretion
flow (RIAF) is expected to occur, when its dimensionless accretion rate is
lower than the critical value. The RIAF is very hot, and it radiates mostly in
the hard X-ray band (>100 keV). Assuming that the accretion disk in every
bright active galactic nucleus (AGN) will finally undergo a RIAF phase while
the accretion rate is lower than the critical value, we calculate the
contribution of the RIAFs in AGNs to the cosmological X-ray background of
10-1000 keV. We find that the timescale of the RIAF accreting at slightly below
the critical rate should be shorter than 0.01 of the bright AGN lifetime if the
critical accretion rate is 0.01, i.e., the accretion rate declines from the
critical value to a rate significantly lower than this critical rate within
this RIAF timescale. The derived RIAF timescale is affected by the parameters
adopted in the model calculations, which is also discussed in this Letter.
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0811.3564
|
Fabrice Martins
|
Properties of WNh stars in the Small Magellanic Cloud: evidence for
homogeneous evolution
|
astro-ph
|
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.
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astro-ph/0401372
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Alice C. Quillen
|
The impact of a close stellar encounter on the Edgeworth-Kuiper Belt
|
astro-ph
|
We numerically investigate the possibility that a close stellar encounter
could account for the high inclinations of the Kuiper belt, as originally
proposed by Ida, Larwood and Burkert, however we consider encounters with
pericenters within those explored by previous works. A star of mass 0.2 Msol
entering the solar system on a nearly parabolic, low inclination, retrograde
orbit with perihelion of 50 AU can scatter ~30% of the Kuiper belt into
moderate inclination (i>10 degrees) and eccentricity orbits, while leaving the
rest of the belt at low eccentricities and inclinations. This scenario provides
a possible explanation for the dichotomy of the Kuiper belt, accounting for the
scattered and classical populations, including objects with high eccentricities
and inclinations. High eccentricity objects that were not placed into resonance
with Neptune are more likely to be removed from the system by subsequent long
timescale evolution. Consequently this scenario suggests that some Plutinos
could have originally been objects in circular orbits which were scattered into
resonant high eccentricity and high inclination orbits by the stellar
encounter. Objects could have been placed into stable resonant regions with
Neptune by the encounter. However long lived regions of dynamical stability
should differ from those selected by resonance capture. This scenario predicts
the presence of objects in resonances with Neptune that are unlikely to have
been captured via Neptune's migration.
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0707.3278
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Riccardo Giovanelli
|
ALFALFA: HI Cosmology in the Local Universe
|
astro-ph
|
For the last 25 years, the 21 cm line has been used productively to
investigate the large-scale structure of the Universe, its peculiar velocity
field and the measurement of cosmic parameters. In February 2005 a blind HI
survey that will cover 7074 square degrees of the high latitude sky was started
at Arecibo, using the 7-beam feed L-band feed array (ALFA). Known as the
Arecibo Legacy Fast ALFA (ALFALFA) Survey, the program is producing a census of
HI-bearing objects over a cosmologically significant volume of the local
Universe. With respect to previous blind HI surveys, ALFALFA offers an
improvement of about one order of magnitude in sensitivity, 4 times the angular
resolution, 3 times the spectral resolution, and 1.6 times the total bandwidth
of HIPASS. ALFALFA can detect 7 X 10**4 D**2 solar masses of HI, where D is the
source distance in Mpc. As of mid 2007, 44% of the survey observations and 15%
of the source extraction are completed. We discuss the status of the survey and
present a few preliminary results, in particular with reference to the proposed
"dark galaxy" VirgoHI21.
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astro-ph/9512011
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Carlton M. Baugh
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The Real Space Correlation Function Measured from the APM Galaxy Survey
|
astro-ph
|
We present a determination of the real space correlation function for
galaxies in the APM Survey, with magnitudes in the range $17 < b_J < 20$. We
recover a power law form for the correlation function $\xi(r) = (r/4.5)^{-1.7}$
on scales $r < 4 Mpc/h$, for a flat universe and clustering that is fixed in
comoving coordinates.If we assume that clustering evolves according to linear
perturbation theory, we obtain $\xi(r) = (r/5.25)^{-1.7}$. We recover a
shoulder feature in $\xi(r)$ for scales $4 < r < 25 Mpc/h$, with the
correlation function rising above the power law, before becoming consistent
with zero around $r > 40 Mpc/h$. The form we obtain for $\xi(r)$ is smooth on
large scales, allowing a determination of the distortions in the correlation
function in redshift space, caused by the peculiar motions of galaxies to be
made. Comparing the real space correlation function with the measurement of the
redshift space correlation function by Loveday et al (1995), we find that a
value for $omega^{0.6}/b$ (where $\omega$ is the density parameter of the
universe and b is the bias factor between the density of light and mass) of
unity is ruled out at the 2 sigma level.
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astro-ph/0406261
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Wei-Hao Wang
|
An 850 Micron SCUBA Survey of the HDF-N GOODS Region
|
astro-ph
|
The Hubble Deep Field-North (HDF-N) is one of the best studied extragalactic
fields, and ultra-deep optical, radio, X-ray, and mid-infrared wide-field
images are available for this area. Here we present an 850 um survey around the
HDF-N, covering most of the area imaged by the Advanced Camera for Surveys as a
part of the Great Observatories Origins Deep Survey. Our map has 0.4-4 mJy
sensitivities (1 sigma) over an area ~110 arcmin^2 and there are 45 sources
detected at >3 sigma. After correcting the effects of noise, confusion,
incompleteness, and the Eddington bias using Monte Carlo simulations, we find
that the detected 850 $\mu$m sources with fluxes greater than 2 mJy have a
surface density of 3200^+1900_-1000 deg^-2 and account for about 24% to 34% of
the far-infrared extragalactic background light. Using the deep radio
interferometric image and the deep X-ray image, we are able to accurately
locate ~60% of the bright submillimeter (submm) sources. In addition, by
assuming the Arp 220 spectral energy distribution in the submm and radio, we
estimate millimetric redshifts for the radio detected submm sources, and
redshift lower limits for the ones not detected in the radio. Using the
millimetric redshifts of the radio identified sources and spectroscopic and
optical photometric redshifts for galaxies around the submm positions, we find
a median redshift of 2.0 for 11 possibly identified sources, or a lower limit
of 2.4 for the median redshift of our 4 sigma sample.
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astro-ph/0203427
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Luca Lamagna
|
MAD-4-MITO, a Multi Array of Detectors for ground-based mm/submm SZ
observations
|
astro-ph
|
The last few years have seen a large development of mm technology and
ultra-sensitive detectors devoted to microwave astronomy and astrophysics. The
possibility to deal with large numbers of these detectors assembled into
multi--pixel imaging systems has greatly improved the performance of microwave
observations, even from ground--based stations, especially combining the power
of multi--band detectors with their new imaging capabilities. Hereafter, we
will present the development of a multi--pixel solution devoted to
Sunyaev--Zel'dovich observations from ground--based telescopes, that is going
to be operated from the Millimetre and Infrared Testagrigia Observatory.
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0801.3267
|
George Rieke
|
Structure of the interstellar medium around Cas A
|
astro-ph
|
We present a three-year series of observations at 24 microns with the Spitzer
Space Telescope of the interstellar material in a 200 x 200 arcmin square area
centered on Cassiopeia A. Interstellar dust heated by the outward light pulse
from the supernova explosion emits in the form of compact, moving features.
Their sequential outward movements allow us to study the complicated
three-dimensional structure of the interstellar medium (ISM) behind and near
Cassiopeia A. The ISM consists of sheets and filaments, with many structures on
a scale of a parsec or less. The spatial power spectrum of the ISM appears to
be similar to that of fractals with a spectral index of 3.5. The filling factor
for the small structures above the spatial wavenumber k ~ 0.5 cycles/pc is only
~ 0.4%.
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astro-ph/9905168
|
John E. Gizis
|
A 2MASS Survey for Brown Dwarfs Towards the Hyades
|
astro-ph
|
We present results of a search for very low mass stars and brown dwarfs
towards the central region of the Hyades cluster. Using Two Micron All Sky
Survey (2MASS) near-infrared and Second Palomar Sky Survey (POSSII) optical
photometry, we select candidates corresponding to spectral types M8 through L4
at the distance of the Hyades. Spectroscopic followup data indicate that none
are cluster members. The lack of members in the mass range 0.08 < M < 0.06
M_solar is consistent with prior evidence that this 600 Myr old cluster is
deficient in very low mass objects. Three of our objects are spectral type M8,
with surface gravity sensitive features intermediate between dwarfs and giants.
We interpret these objects as background brown dwarfs, perhaps associated with
the Taurus star formation region.
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astro-ph/0612052
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Claudio M. G. de Sousa
|
Rotating Boson and Boson-Fermion Stars
|
astro-ph
|
Some recent results on rotating self-gravitating configurations composed with
Bosons and Fermions are reported. Given a star composed of both Bosons and
Fermions without interaction, it is shown that it is possible to obtain stable
slowly rotating configurations by using the same perturbative relativistic
method that usually describes neutron stars.
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astro-ph/9906356
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John Blakeslee
|
Globular Clusters in Dense Clusters of Galaxies
|
astro-ph
|
Deep imaging data from the Keck II telescope are employed to study the
globular cluster (GC) populations in the cores of six rich Abell clusters. The
sample includes A754, A1644, A2124, A2147, A2151, and A2152, and spans the
redshift range z = 0.035-0.066. The clusters also range in morphology from
spiral-rich, irregular systems to centrally concentrated cD clusters rich in
early-type galaxies. Globular cluster specific frequencies S_N and luminosity
function dispersions are measured for a total of 9 galaxies in six central
fields. The measured values of S_N for the six brightest cluster galaxies
(BCGs) are all higher than typical values for giant ellipticals, in accord with
the known S_N-density correlations. The three non-BCGs analyzed also have
elevated values of S_N, confirming that central location is a primary factor.
The number of GCs per unit mass for these fields are consistent with those
found in an earlier sample, giving further evidence that GC number scales with
mass and that the S_N variations are due to a deficit of halo light, i.e., S_N
reflects mass-to-light ratio.
The discussion builds on an earlier suggestion that the GCs (both metal rich
and metal poor) around the central cluster galaxies were assembled at early
times, and that star formation halted prematurely in the central galaxies at
the epoch of cluster collapse. This is consistent with recent simulations of
BCG/cluster formation. The subsequent addition of luminous material through
cluster dynamical evolution can cause S_N to decrease, and we may be seeing the
first evidence of this. Finally, the GC luminosity function measurements are
used to constrain the relative distances of the three clusters that make up the
Hercules supercluster.
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0804.2392
|
Carrie Trundle Dr
|
SN 2005 gj: Evidence for LBV supernovae progenitors?
|
astro-ph
|
There has been mounting observational evidence in favour of Luminous Blue
Variables (LBVs) being the direct progenitors of supernovae. Here we present
possibly the most convincing evidence yet for such progenitors. We find
multiple absorption component P-Cygni profiles of hydrogen and helium in the
spectrum of SN 2005gj, which we interpret as being an imprint of the
progenitors mass-loss history. Such profiles have previously only been detected
in Luminous Blue Variables. This striking resemblance of the profiles, along
with wind velocities and periods consistent with LBV's leads us to connect SN
2005gj to an LBV progenitor.
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0808.2317
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Vasily Zakharov
|
Evidence of convective rolls in a sunspot penumbra
|
astro-ph
|
aims: We study the recently discovered twisting motion of bright penumbral
filaments with the aim of constraining their geometry and the associated
magnetic field. methods: A large sunspot located $40\degr$ from disk center was
observed at high resolution with the 1-m Swedish Solar Telescope. Inversions of
multi-wavelength polarimetric data and speckle reconstructed time series of
continuum images were used to determine proper motions, as well as the velocity
and magnetic structure in penumbral filaments. results: The continuum movie
reveals apparent lateral motions of bright and dark structures inside bright
filaments oriented parallel to the limb, confirming recent Hinode results. In
these filaments we measure upflows of $\approx 1.1 \mathrm{km/s}$ on their
limbward side and weak downflows on their centerward side. The magnetic field
in them is significantly weaker and more horizontal than in the adjacent dark
filaments. conclusions: The data indicate the presence of vigorous convective
rolls in filaments with a nearly horizontal magnetic field. These are separated
by filaments harbouring stronger, more vertical fields. Because of reduced gas
pressure, we see deeper into the latter. When observed near the limb, the
disk-centerward side of the horizontal-field filaments appear bright due to the
\textit{hot wall} effect known from faculae. We estimate that the convective
rolls transport most of the energy needed to explain the penumbral radiative
flux.
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astro-ph/0006153
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Douglas L. Tucker
|
Loose Groups of Galaxies in the Las Campanas Redshift Survey
|
astro-ph
|
A ``friends-of-friends'' percolation algorithm has been used to extract a
catalogue of dn/n = 80 density enhancements (groups) from the six slices of the
Las Campanas Redshift Survey (LCRS). The full catalogue contains 1495 groups
and includes 35% of the LCRS galaxy sample. A clean sample of 394 groups has
been derived by culling groups from the full sample which either are too close
to a slice edge, have a crossing time greater than a Hubble time, have a
corrected velocity dispersion of zero, or contain a 55-arcsec ``orphan'' (a
galaxy with a mock redshift which was excluded from the original LCRS redshift
catalogue due to its proximity to another galaxy -- i.e., within 55 arcsec).
Median properties derived from the clean sample include: line-of-sight velocity
dispersion sigma_los = 164km/s, crossing time t_cr = 0.10/H_0, harmonic radius
R_h = 0.58/h Mpc, pairwise separation R_p = 0.64/h Mpc, virial mass M_vir =
(1.90x10^13)/h M_sun, total group R-band luminosity L_tot = (1.30x10^11)/h^2
L_sun, and R-band mass-to-light ratio M/L = 171h M_sun/L_sun; the median number
of observed members in a group is 3.
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astro-ph/0209457
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Judith G. Cohen
|
The Redshift of the Lensed Object in the Einstein Ring B0218+357
|
astro-ph
|
We present a secure redshift of z=0.944\pm0.002 for the lensed object in the
Einstein ring gravitational lens B0218+357 based on five broad emission lines,
in good agreement with our preliminary value announced several years ago based
solely on the detection of a single emission line.
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astro-ph/0302026
|
Lamzin S. A.
|
Possible detection of a magnetic field in T Tauri
|
astro-ph
|
Medium-resolution $(R\simeq 15000)$ circular spectropolarimetry of T Tauri is
presented. The star was observed twice: on November 11, 1996 and January 22,
2002. Weak circular polarization has been found in photospheric absorption
lines, indicating a mean surface longitudinal magnetic field $B_{\|}$ of
$160\pm 40$ G and $140\pm 50$ G at the epoch of the first and second
observations respectively. While these values are near the detection limit of
our apparatus, we belive that they are real. In any case one can conclude from
our data that $B_{\|}$ of T Tau does not significantly exceed 200 G, which is
much less than surface magnetic field strength of the star ($>2.3$ kG) found by
Guenther et al. (1999) and Johns-Krull et al. (2000). We discuss possible
reasons of this difference.
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astro-ph/9610052
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Mario Gambera
|
Substructure effects on the collapse of density perturbations
|
astro-ph
|
We solve numerically the equations of motion for the collapse of a shell of
baryonic matter, made of galaxies and substructure of $ 10^{6} M_{\odot}-
10^{9} M_{\odot}$, falling into the central regions of a cluster of galaxies
taking account of dynamical friction. We calculate the evolution of the
expansion parameter, a(t), of the perturbation using a coefficient of dynamical
friction, $ \eta_{0}$, calculated for a perturbation in which clustering is
absent and a coefficient $ \eta_{cl}$ obtained from a clustered one. The effect
of the dynamical friction is to slow down the collapse (V. Antonuccio-Delogu \&
S. Colafrancesco 1994, hereafter AC) producing an observable variation of the
parameter of expansion of the shell. The effect increases with increasing $
\eta $ and with the increasing of clustering. Finally, we show how the collapse
time depends on $ \eta_{0}$ and $ \eta_{cl}$. keywords: {cosmology: theory-
galaxies: formation}
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astro-ph/0609388
|
Faustin Munyaneza
|
Degenerate sterile neutrino dark matter in the cores of galaxies
|
astro-ph
|
We study the distribution of fermionic dark matter at the center of galaxies
using NFW, Moore and isothermal density profiles and show that dark matter
becomes degenerate for particle masses of a few {\rm keV} and for distances
less than a few parsec from the center of our galaxy. A compact degenerate core
forms after galaxy merging and boosts the growth of supermassive black holes at
the center of galaxies. To explain the galactic center black hole of mass of
$\sim 3.5 \times 10^{6}M_{\odot}$ and a supermassive black hole of $\sim 3
\times 10^{9}M_{\odot}$ at a redshift of 6.41 in SDSS quasars, we require a
degenerate core of mass between $3 \times 10^{3} M_{\odot}$ and $3.5 \times
10^{6}M_{\odot}$. This constrains the mass of the dark matter particle between
$0.6 {\rm keV}$ and $82 {\rm keV}$. The lower limit on the dark matter mass is
improved to {\rm 7 keV} if exact solutions of Poisson's equation are used in
the isothermal power law case. We argue that the constrained particle could be
the long sought dark matter of the Universe that is interpreted here as a
sterile neutrino.
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0708.1438
|
Athina Meli
|
Particle acceleration in relativistic subluminal shock environments
|
astro-ph
|
The understanding of the particle spectra resulting from acceleration in
relativistic shocks as they occur in extragalactic sources, is essential for
the interpretation of the cosmic ray spectrum above the ankle ($E_p>3\cdot
10^{18}$ eV). It is believed that extragalactic sources like Active Galactic
Nuclei and Gamma Ray Bursts can produce particle spectra up to $E_p\sim
10^{21}$ eV. In this contribution, subluminal shocks are investigated with
respect to different shock boost factors $\Gamma$ and the inclination angle
between the shock normal and the magnetic field $\psi$. A correlation between
the boost factor and the spectral behavior of the emitted particles is found.
The results are compared to Active Galactic Nuclei and Gamma Ray Burst diffuse
cosmic ray contribution and the observed cosmic ray spectrum at the highest
energies.
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astro-ph/0001445
|
JieHao Huang
|
The FIR-Radio Correlation of Wolf-Rayet Galaxies and the Role of Star
Formation in LINERs
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astro-ph
|
We find that a preliminary classification of LINERs' energetics may be made
in terms of the FIR-radio correlation of Wolf-Rayet galaxies. The AGN- or
starburst-supported LINERs can be distinguished by their FIR-to-radio ratio,
$Q\equiv L(1.4{\rm GHz})/$ $L(60\mu{\rm m})>$ or $<0.01$. It is interesting to
note that almost all the LINERs with inner rings might be starburst- supported,
indicating reduced AGN activities compared with those of the AGN-supported
ones. We also find that a shock-heating phase for the warm dust component might
be important for some starbursts at the burst age of $\ge 10^{7}$ yr, with
$Q<0.001$.
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astro-ph/9510005
|
Sandip Chakrabarti
|
Spectral Properties of Accretion Disks Around Galactic and Extragalactic
Black Holes
|
astro-ph
|
We study the spectral properties of a very general class of accretion disks
which can be decomposed into three distinct components apart from a shock at
$r=r_s$: (1) An optically thick Keplerian disk on the equatorial plane
($r>r_s$), (2) A sub-Keplerian optically thin halo above and below this
Keplerian disk $r>r_s$ and (3) A hot, optically slim, $\tau\sim 1$ postshock
region $r<r_s\sim 5-10 r_g$ where $r_g$ is the Schwarzschild radius. The
postshock region intercepts soft photons from the Keplerian component and
reradiates them as hard X-rays and $\gamma$ rays after Comptonization. We solve
two-temperature equations in the postshock region with Coulomb energy exchange
between protons and electrons, and incorporating radiative processes such as
bremsstrahlung and Comptonization. We also present the exact prescription to
compute the reflection of the hard X-rays from the cool disk. We produce
radiated spectra from both the disk components as functions of the accretion
rates and compare them with the spectra of galactic and extragalactic black
hole candidates. We find that the transition from hard state to soft state is
smoothly initiated by a single parameter, namely, the mass accretion rate of
the disk. In the soft state, when the postshock region is very optically thick
and cooled down, bulk motion of the converging flow determines the spectral
index to be about $1.5$ in agreement with observations.
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astro-ph/9505047
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Thierry Contini
|
Starbursts in barred spiral galaxies. I. Mrk 712: a new Wolf-Rayet
galaxy
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astro-ph
|
We report the discovery of emission from Wolf-Rayet stars in a giant HII
region 4.5 arcsec South of the nucleus of the IRAS barred spiral galaxy Mrk
712. The ratio of WNL to OV stars, estimated from the luminosity of the HeII
4686 line, is 0.2. By comparison with starburst and stellar evolution models,
we find that this high value is only compatible with a very young starburst
episode (3 - 4 Myr) and a flat initial mass function (Gamma = -1). The presence
of the [ArV] line reveals that the HII region is strongly ionized by the hot
Wolf-Rayet stars. The comparison with other barred Wolf-Rayet galaxies suggests
that the detection of Wolf-Rayet stars depends on the dust content and
orientation of the galaxy.
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astro-ph/0308509
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Thomas M. Brown
|
The UV Upturn: From M32 to Distant Clusters
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astro-ph
|
I review the observational constraints on the stars responsible for the
upturn in the UV spectra of ellipticals, ranging from galaxies in the local
Universe to distant clusters. In nearby galaxies, this UV upturn is produced by
a minority population of extreme horizontal branch (EHB) stars, with the large
variations observed in the UV-to-optical flux ratio driven by variations in the
number of EHB stars, and not the type of UV-bright stars. Deep UV images of the
nearest elliptical galaxy, M32, show that it has a well-populated EHB, even
though it has the weakest UV upturn of any known elliptical galaxy. However,
M32 suffers from a striking dearth of the hot post-HB stars expected from
canonical evolutionary theory. As we observe to larger lookback times in more
distant galaxy clusters, the UV upturn fades, as predicted by theories of
stellar and galactic evolution, but does so gradually. Because the EHB stars do
not appear suddenly in the Universe, their presence is likely driven by a large
dispersion in the parameters that govern HB morphology.
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astro-ph/0510424
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Franck Galland
|
Extrasolar planets and brown dwarfs around A-F type stars III.
Beta-Pictoris : looking for planets, finding pulsations
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astro-ph
|
In the frame of the search for extrasolar planets and brown dwarfs around
early-type stars, we present the results obtained on Beta-Pictoris, which is
surrounded by a circumstellar disk that is warped by the presence of a planet.
We used 97 spectra acquired with CORALIE and 230 spectra acquired with HARPS to
characterize the radial velocity behavior of Beta-Pictoris and to infer
constraints on the presence of a planet close to this star. With these data, we
were able to exclude the presence of an inner giant planet (2 MJup at a
distance to the star of 0.05 AU, 9 MJup at 1 AU). We also discuss the origin of
the observed radial velocity variations in terms of Delta-Scuti type
pulsations.
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No Label
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No Label
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0709.2533
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Kari Nilsson
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Host galaxy subtraction of TeV candidate BL Lacertae objects
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astro-ph
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We have derived R-band host galaxy fluxes of 20 TeV candidate BL Lacertae
objects as a function of aperture radius and FWHM. The results are given as
correction tables, that list the fluxes (in mJy) of all ``contaminating''
sources (host galaxy + significant nearby objects) as a function of aperture
radius and FWHM. We found that the derived fluxes depend strongly on aperture
radius, but the FWHM has only a minor effect (a few percent). We also discuss
the implications of our findings to optical monitoring programs and potential
sources of error in our derived fluxes. During this work we have also
constructed new calibration star sequences for 9 objects and present the
finding charts and calibrated magnitudes.
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No Label
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astro-ph/0202146
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Marcella Contini
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The signature of high velocity gas in the spectra of NGC 4151
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astro-ph
|
The multiwavelength emission spectrum and associated velocity field of the
Seyfert prototype NGC 4151 is modeled. HST data has impressively revealed the
existence of a large range of velocities (100 - 1500 \kms) dominating the
emitting clouds in the extended emission line region of the galaxy. Following
this observational result, a revision of the photoionization modeling approach
applied to NGC 4151 is presented. It is concluded that a mixture of radiation
dominated clouds and shock dominated clouds are required to explain the
multiwavelength line and continuum spectra of the galaxy.
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No Label
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astro-ph/0204402
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Vladimir V. Usov
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Two-stream Instability in Pulsar Magnetospheres
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astro-ph
|
Creation of electron-positron pairs near the pulsar surface and the
parameters of plasma in pulsar magnetospheres are discussed. It is argued that
the pair creation process is nonstationary, and the pair plasma that flows out
from the pulsar environment is strongly nonhomogeneous and gathers into
separate clouds. Plasma instabilities in the outflowing plasma are reviewed.
The two-stream instability that develops due to strong nonhomogeneity of the
outflowing plasma is the most plausible reason for the generation of coherent
radio emission of pulsars. The development of the two-stream instability in
pulsar magnetospheres is considered.
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astro-ph/9309053
| null |
The metal systems in Q0000--2619 at high resolution
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astro-ph
|
We have obtained high, 11 and 14 \kms, and medium, 40 and 53 \kms, resolution
spectra of the $z_{em} = 4.11$ quasar Q0000--2619 covering the range 4400 \AA\
to 9265 \AA . We identify nine metal absorption systems, of which four were
previously known. A fifth previously suggested system at $z_{abs} \approx
3.409$ (Turnshek et al~ 1991) is ruled out by our data. Two of the eight
systems for which the \lya~ line is in the observable range have a damped \lya~
line. Six of the nine systems show evidence for complex sub--component
structure. At our resolution and S/N we identify a total of 21 sub--components
in the nine systems. Five of the nine systems (11 of the 21 components) fall
within the $\pm 5000$ \kms~ range of the emission redshift, and are hence
classified as \zae~ absorbers. For the two damped systems we find metal
abundances of $\leq 1$% and $\leq 8$% of solar values at redshifts of 3.0541
and 3.3901 respectively. These upper limits are consistent with what would be
expected from previous determinations at lower redshifts, and our data are
hence compatible with earlier conclusions that no evidence is yet found for
chemical evolution of intervening damped and Lyman limit absorbers. For the
\zae~ systems we found indications of metallicities comparable to, and even in
excess of solar values. These much higher values compared to the damped
systems, are in favour of the intrinsic hypothesis for these systems.
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astro-ph/0110096
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Lori E. Allen
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HST/NICMOS Imaging Survey of the Ophiuchus (Lynds 1688) Cluster
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astro-ph
|
We present a catalogue of near-infrared photometry of young stars associated
with the Ophiuchus molecular cloud, based on observations made with the Hubble
Space Telescope NICMOS-3 camera at 1.1 $\mu$m and 1.6 $\mu$m. Our survey covers
0.02 square degrees centered on the dense molecular cores in Lynds 1688. We
detect 165 sources at 1.6 $\mu$m and 65 sources at 1.1 $\mu$m, within our
estimated completeness limits of 21.0 mag and 21.5 mag, respectively. An
analysis of the cloud extinction, based on existing molecular line maps,
suggests that most of the sources lying within the 40 $\rm{A_V}$ extinction
contour of the cloud are probable cloud members. Approximately half (58/108) of
these sources are previously unpublished. The faint embedded sources revealed
by these observations are spatially concentrated in three regions of high
stellar space density (N$> 10^4$ stars pc$^{-3}$). While the spatial
distribution of these sources reflects that of the brighter, well--known
population of young stars in Ophiuchus, it is distinctly different from the
distribution of cool concentrations seen in the submillimeter. Seven new brown
dwarf candidates are identified, based on their infrared colors and their
projected locations on high column-density regions of the molecular cloud.
Eight new candidate binary and five new candidate triple systems, having
separations between 0\farcs2 to 10\arcsec (29 to 1450 AU) are reported. The
spatial resolution and sensitivity of these observations reveal five apparent
disk/envelope systems seen via scattered light, and four nebulous objects with
complex morphologies.
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astro-ph/0505493
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Seiji Zenitani
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Three Dimensional Evolution of a Relativistic Current Sheet : Triggering
of Magnetic Reconnection by the Guide Field
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astro-ph
|
The linear and non-linear evolution of a relativistic current sheet of pair
($e^{\pm}$) plasmas is investigated by three-dimensional particle-in-cell
simulations. In a Harris configuration, it is obtained that the magnetic energy
is fast dissipated by the relativistic drift kink instability (RDKI). However,
when a current-aligned magnetic field (the so-called "guide field") is
introduced, the RDKI is stabilized by the magnetic tension force and it
separates into two obliquely-propagating modes, which we call the relativistic
drift-kink-tearing instability (RDKTI). These two waves deform the current
sheet so that they trigger relativistic magnetic reconnection at a crossover
thinning point. Since relativistic reconnection produces a lot of non-thermal
particles, the guide field is of critical importance to study the energetics of
a relativistic current sheet.
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No Label
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astro-ph/0507194
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Gamil Cassam-Chenai
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Morphology of synchrotron emission in young supernova remnants
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astro-ph
|
In the framework of test-particle and cosmic-ray modified hydrodynamics, we
calculate synchrotron emission radial profiles in young ejecta-dominated
supernova remnants (SNRs) evolving in an ambient medium which is uniform in
density and magnetic field. We find that, even without any magnetic field
amplification by Raleigh-Taylor instabilities, the radio synchrotron emission
peaks at the contact discontinuity because the magnetic field is compressed and
is larger there than at the forward shock. The X-ray synchrotron emission
sharply drops behind the forward shock as the highest energy electrons suffer
severe radiative losses.
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No Label
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astro-ph/0203393
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Marcio Catelan
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M75, a Globular Cluster with a Trimodal Horizontal Branch. I.
Color-Magnitude Diagram
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astro-ph
|
Deep UBVI photometry for a large field covering the distant globular cluster
M75 (NGC 6864) is presented. We confirm a previous suggestion (Catelan et al.
1998a) that M75 possesses a bimodal horizontal branch (HB) bearing striking
resemblance to the well-known case of NGC 1851. In addition, we detect a third,
smaller grouping of stars on the M75 blue tail, separated from the bulk of the
blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may
correspond to the upper part of a very extended, though thinly populated, blue
tail. Thus M75 appears to have a trimodal HB. The presence of the "Grundahl
jump" is verified using the broadband U filter. We explore the color-magnitude
diagram of M75 with the purpose of deriving the cluster's fundamental
parameters, and find a metallicity of [Fe/H] = -1.03 +/- 0.17 dex and -1.24 +/-
0.21 dex in the Carretta & Gratton (1997) and Zinn & West (1984) scales,
respectively. We discuss earlier suggestions that the cluster has an
anomalously low ratio of bright red giants to HB stars. A differential age
analysis with respect to NGC 1851 suggests that the two clusters are
essentially coeval.
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astro-ph/0305203
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Kentaro Nagamine
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The Probability Distribution Function of Light in the Universe: Results
from Hydrodynamic Simulations
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astro-ph
|
While second and higher order correlations of the light distribution have
received extensive study, the lowest order probability distribution function
(PDF) -- the probability that a unit volume of space will emit a given amount
of light -- has received very little attention. We estimate this function with
the aid of hydrodynamic simulations of the Lambda CDM model, finding it
significantly different from the mass density PDF, and not simply related to it
by linear bias or any of the other prescriptions commonly adopted. If the
optical light PDF is, in reality, similar to what we find in the simulations,
then some measures of Omega_M based on mass-to-light ratio and the cosmic
virial theorem will have significantly underestimated Omega_M. Basically, the
problem is one of selection bias, with galaxy forming regions being
unrepresentative of the dark matter distribution in a way not described by
linear bias. Knowledge of the optical PDF and the plausible assumption of a
log-normal distribution for the matter PDF will allow one to correct for these
selection biases. We find that this correction (which amounts to 20-30%) brings
the values of Omega_M estimated by using the mass-to-light ratio and the cosmic
virial theorem to the range 0.2-0.3, in better agreement with the WMAP result
than the uncorrected estimates. In addition, the relation between mass and
light PDFs gives us insight concerning the nature of the void phenomenon. In
particular our simulation indicates that 20% of mass is distributed in voids,
which occupy 85% of volume in the universe.
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astro-ph/0502488
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Giancarlo Ghirlanda
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The peak luminosity - peak energy correlation in GRBs
|
astro-ph
|
We derive the peak luminosity - peak energy (L_iso - E_peak) correlation
using 22 long Gamma-Ray Bursts (GRBs) with firm redshift measurements. We find
that its slope is similar to the correlation between the time integrated
isotropic emitted energy E_iso and E_peak (Amati et al. 2002). For the 15 GRBs
in our sample with estimated jet opening angle we compute the collimation
corrected peak luminosity L_gamma, and find that it correlates with E_peak.
This has, however, a scatter larger than the correlation between E_peak and
E_gamma (the time integrated emitted energy, corrected for collimation;
Ghirlanda et al. 2004), which we ascribe to the fact that the opening angle is
estimated through the global energetics. We have then selected a large sample
of 442 GRBs with pseudo--redshifts, derived through the lag-luminosity
relation, to test the existence of the L_iso-E_peak correlation. With this
sample we also explore the possibility of a correlation between time resolved
quantities, namely L_iso,p and the peak energy at the peak of emission
E_peak,p.
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0706.3368
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Amaya Moro-Martin
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The dust, planetesimals and planets of HD 38529
|
astro-ph
|
HD 38529 is a post-main sequence G8III/IV star (3.5 Gyr old) with a planetary
system consisting of at least two planets having Msin(i) of 0.8 MJup and 12.2
MJup, semimajor axes of 0.13 AU and 3.74 AU, and eccentricities of 0.25 and
0.35, respectively. Spitzer observations show that HD 38529 has an excess
emission above the stellar photosphere, with a signal-to-noise ratio (S/N) at
70 micron of 4.7, a small excess at 33 micron (S/N=2.6) and no excess <30
micron. We discuss the distribution of the potential dust-producing
planetesimals from the study of the dynamical perturbations of the two known
planets, considering in particular the effect of secular resonances. We
identify three dynamically stable niches at 0.4-0.8 AU, 20-50 AU and beyond 60
AU. We model the spectral energy distribution of HD 38529 to find out which of
these niches show signs of harboring dust-producing plantesimals. The secular
analysis, together with the SED modeling resuls, suggest that the planetesimals
responsible for most of the dust emission are likely located within 20-50 AU, a
configuration that resembles that of the Jovian planets + Kuiper Belt in our
Solar System. Finally, we place upper limits (8E-6 lunar masses of 10 micron
particles) to the amount of dust that could be located in the dynamically
stable region that exists between the two planets (0.25--0.75 AU).
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astro-ph/0001389
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Charles Dermer
|
External Shock Model for Gamma-Ray Bursts during the Prompt Phase
|
astro-ph
|
The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be
explained by an external shock model where a single relativistic blast wave
interacts with the surrounding medium. Besides reproducing the generic spectral
behavior of GRB profiles, the external shock model provides quantitative fits
to the peak flux distribution, the > 1 s t_{50} duration distribution, and the
distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE.
The apparent paradox between a relativistic beaming scenario and the empirical
finding that E_pk values are preferentially measured within the triggering
range of a GRB detector is resolved by this model when blast wave physics and
detector triggering criteria are taken into account. Some surprising
implications follow, namely that the fireball event rate is ~1 per 10^4 years
per Milky Way galaxy for unbeamed sources, and proportionally more if fireball
outflows are collimated. This is ~ 3 orders of magnitude larger than normally
quoted. Most of the clean and dirty fireball transients are undetected due to
telescope sensitivity and design limitations.
Strongly variable GRB time histories with good radiative efficiencies are
possible because of the strongly enhanced emissions when a blast wave interacts
with density inhomogeneities located nearly along the line-of-sight to the
observer. Arguments against short timescale variability in an external shock
model are answered, and difficulties in an internal shock/colliding shell model
are mentioned.
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astro-ph/9907088
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Monica Colpi
|
Dynamical friction and the evolution of satellites in virialized halos:
the theory of linear response
|
astro-ph
|
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.
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No Label
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astro-ph/0603167
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Kevin Blagrave
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A photoionized Herbig-Haro object in the Orion nebula
|
astro-ph
|
The spectra of Herbig Haro objects are usually characteristic of ionization
and excitation in shock-heated gas, whether an internal shock in an unsteady
outflow or a bow shock interface with the interstellar medium. We examine the
eastern-most shock -- the leading optically visible shock -- of a Herbig Haro
outflow (HH 529) seen projected on the face of the Orion Nebula, using deep
optical echelle spectroscopy, showing that the spectrum of this gas is
consistent with photoionization by $\theta^1$ Ori C. By modeling the emission
lines, we determine a gas-phase abundance of Fe which is consistent with the
depleted (relative to solar) abundance found in the Orion nebula -- evidence
for the presence of dust in the nebula and therefore in the Herbig Haro
outflow. The spectrum also allows for the calculation of temperature
fluctuations, $t^2$, in the nebula and the shock. These fluctuations have been
used to explain discrepancies between abundances obtained from recombination
lines versus those obtained from collisionally-excited lines, although to date
there has not been a robust theory for how such large fluctuations ($t^2 >
0.02$) can exist.
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astro-ph/0204149
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Diego Herranz
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Scale-Adaptive filters for the detection/separation of compact sources
|
astro-ph
|
This paper presents scale-adaptive filters that optimize the
detection/separation of compact sources on a background. We assume that the
sources have a multiquadric profile, i. e. $\tau (x) = {[1 +
{(x/r_c)}^2]}^{-\lambda}, \lambda \geq {1/2}, x\equiv |\vec{x}|$, and a
background modeled by a homogeneous and isotropic random field, characterized
by a power spectrum $P(q)\propto q^{-\gamma}, \gamma \geq 0, q\equiv
|\vec{q}|$. We make an n-dimensional treatment but consider two interesting
astrophysical applications related to clusters of galaxies (Sunyaev-Zel'dovich
effect and X-ray emission).
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No Label
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astro-ph/0606243
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Etienne Artigau
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C stars in the outer spheroid of NGC 6822
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astro-ph
|
From a 2 x 2 degree survey of NGC 6822 we have previously established that
this Local Group dwarf irregular galaxy possesses a huge spheroid having more
than one degree in length. This spheroid is in rotation but its rotation curve
is known only within ~15' from the center. It is therefore critical to identify
bright stars belonging to the spheroid to characterize, as far as possible, its
outer kinematics. We use the new wide field near infrared imager CPAPIR,
operated by the SMARTS consortium, to acquire J, Ks images of two 34.8' x 34.8'
areas in the outer spheroid to search for C stars. The colour diagram of the
fields allows the identification of 192 C stars candidates but a study of the
FWHM of the images permits the rejection of numerous non-stellar objects with
colours similar to C stars. We are left with 75 new C stars, their mean Ks
magnitude and mean colour are similar to the bulk of known NGC 6822 C stars.
This outer spheroid survey confirms that the intermediate-age AGB stars are a
major contributor to the stellar populations of the spheroid. The discovery of
some 50 C stars well beyond the limit of the previously known rotation curve
calls for a promising spectroscopic follow-up to a major axis distance of 40'.
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astro-ph/0605146
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Masahiro Machida N
|
The First Jet in the Universe: Protostellar Jets from the First Stars
|
astro-ph
|
The protostellar jets driven by the formation of the first stars are studied
by using MHD nested grid simulations. Starting from a slowly rotating spherical
cloud of 5.1 times 10^4 Msun permeated by a uniform magnetic field, we follow
the evolution from the central number density n = 10^3 cm^-3 to n simeq 10^23
cm^-3. We calculate four models that differ in initial magnetic field strengths
and angular velocities. In all models, protostars of simeq 10^-3 Msun are
formed at n simeq 10^22 cm^-3 in accordance with one-dimensional calculations.
By this epoch, the magnetic flux density is amplified by 10 orders of magnitude
from the initial value. Consequently, the formed protostar possesses the
magnetic field of \sim 10^6 G that is much larger than the flux density of the
present counterparts, reflecting the fact that the dissipation of a magnetic
field is ineffective in primordial gas clouds. If the initial magnetic field B
> 10^-9 (n/10^3 cm^-3)^2/3 G, the protostellar jet is launched and its
velocities reaches \sim 70 km s^-1 by the time the protostellar mass becomes
(4-6) times 10^-3 Msun, and a fraction (3-10%) of the accreting matter is blown
off from the central region. Owing to the interaction of these ejecta with
surrounding matter, expanding bow shocks are created at both heads of the jet.
If this jet continues to sweep out the surrounding gas that otherwise accretes
onto the central star or circumstellar disk, the final mass of the first star
can be substantially reduced. In addition, dense post-shock regions behind the
bow shocks are expected to promote the chemical reactions (formation of H_2 and
HD), and this provides possible environments for subsequent low-mass star
formation in the early universe.
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astro-ph/9707150
|
Rob Swaters
|
The HI Halo of NGC 891
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astro-ph
|
Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891
reveal the presence of an HI halo extending up to at least 5 kpc from the
plane. This halo gas appears to rotate 25 to 100 km/s more slowly than the gas
in the plane. If this velocity difference is due to the gradient in the
gravitational potential, then it may serve to discriminate between disk and
spheroidal mass models. The classic picture of a large outer flare in the HI
layer of NGC 891 may no longer be valid. A correlation is seen between the
distributions of HI, Halpha and radio continuum emission, which supports, in
accordance with galactic fountain models, the picture of a substantial
disk-halo circulation related to the star formation activity in the disk of NGC
891. There is now also clear evidence for the presence of a rapidly rotating
(v_rot ~ 230 km/s) disk or ring of HI in the central part of NGC 891.
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astro-ph/0402594
|
Francesca de Angeli
|
The ubiquitous nature of the Horizontal Branch second U-jump: A link
with the Blue Hook scenario?
|
astro-ph
|
In a previous paper we reported on a discontinuity in the extreme horizontal
branch (EHB) of the Galactic globular cluster NGC6752, which we called the
second U-jump. This feature was attributed to a combination of post zero-age
horizontal branch evolution and diffusion effects. In this follow-up study we
analyze other EHB clusters and show that the second U-jump is a common feature
among EHB clusters reaching T_{eff}\ge 23,000K, and that its onset in different
clusters converges around T_{eff}\sim 21,000\pm3,000K. We also present
near-ultraviolet diagrams of \omega Cen and NGC2808, the only two objects with
spectroscopically confirmed ``blue hook'' stars (T_{eff}\ge 35,000K). We
confirm predictions of a photometric discontinuity separating late from
early-helium flashers. Moreover, we present empirical evidence that the second
U-jump population might be mainly composed by early-helium flashers. Lastly, we
revisit the discussion on the ubiquitous nature of the gaps and jumps so far
identified in the blue HB tails, suggesting a possible discrete nature of the
distribution in temperature of the HB stars.
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astro-ph/0012276
|
EAprile Group
|
An Upgraded Data Acquisition System for the Balloon-Borne Liquid Xenon
Gamma-Ray Telescope LXeGRIT
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astro-ph
|
LXeGRIT is a balloon-borne Compton telescope for MeV gamma-ray astrophysics,
based on a liquid xenon time projection chamber (LXe-TPC) with charge and light
readout. The first balloon flights in 1997 revealed limitations of the trigger
electronics and the data acquisition (DAQ) system, leading to their upgrade.
New electronics was developed to handle the xenon scintillation light trigger.
The original processor module was replaced by a commercial VME processor. The
telemetry rate was doubled to 2 x 500 kbps and onboard data storage on hard
disks was implemented. Relying on a robust real-time operating system, the new
DAQ software adopts an object oriented design to implement the diverse tasks of
trigger handling, data selection, transmission, and storage, as well as DAQ
control and monitor functions. The new systems performed well during two
flights in Spring 1999 and Fall 2000. In the 2000 flight, the DAQ system was
able to handle 300-350 triggers/s out of a total of about 650 Hz, including
charged particles.
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astro-ph/0409551
|
David Radburn-Smith
|
A Comparison of Local SNIa with the IRAS PSCz Gravity Field
|
astro-ph
|
We compare the measured peculiar velocities of 98 local (<150/h Mpc) type Ia
supernovae with predictions derived from the PSCz survey. There is excellent
agreement between the two datasets with a best fit beta_I (=Omega_m^0.6/b_I) of
0.55+-0.06. Subsets of the supernovae dataset are further analysed and the
above result is found to be robust with respect to culls by distance,
host-galaxy extinction and to the choice of reference frame in which the
analysis is carried out. Alternative methods of determining beta_I including
density-density comparisons, dipole measurements and WMAP-based results are
also discussed. We conclude that most recent determinations are consistent with
a value of beta_I=0.5.
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astro-ph/0512037
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Henrik Spoon
|
Spitzer Observations of Deeply Obscured Galactic Nuclei
|
astro-ph
|
We report on our first results from a mid-infrared spectroscopic study of ISM
features in a sample of deeply obscured ULIRG nuclei using the InfraRed
Spectrograph (IRS) on the Spitzer Space Telescope. The spectra are extremely
rich and complex, revealing absorption features of both amorphous and
crystalline silicates, aliphatic hydrocarbons, water ice and gas phase bands of
hot CO and warm C_2H_2, HCN and CO_2. PAH emission bands were found to be
generally weak and in some cases absent. The features are probing a dense and
warm environment in which crystalline silicates and water ice are able to
survive but volatile ices, commonly detected in Galactic dense molecular
clouds, cannot. If powered largely by star formation, the stellar density and
conditions of the gas and dust have to be extreme not to give rise to the
commonly detected emission features associated with starburst.
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astro-ph/0208347
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Sheila McBreen
|
Cumulative light curves of gamma-ray bursts and relaxation systems
|
astro-ph
|
The cumulative light curves of a large sample of gamma-ray bursts (GRBs) were
obtained by summing the BATSE counts. The smoothed profiles are much simpler
than the complex and erratic running light curves that are normally used. For
most GRBs the slope of the cumulative light curve (S) is approximately constant
over a large fraction of the burst. The bursts are modelled as relaxation
systems that continuously accumulate energy in the reservoir and
discontinuously release it. The slope is a measure of the cumulative power
output of the central engine. A plot of S versus peak flux in 64ms (P64ms)
shows a very good correlation over a wide range for both short and long GRBs.
No relationship was found between S and GRBs with known redshift. The standard
slope (S'), which is representative of the power output per unit time, is
correlated separately with P64ms for both sub-classes indicating more powerful
outbursts for the short GRBs. S' is also anticorrelated with GRB duration.
These results imply that GRBs are powered by accretion into a black hole.
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astro-ph/0603207
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Andrea Martocchia
|
The XMM-Newton view of GRS1915+105 during a "plateau"
|
astro-ph
|
Two XMM-Newton observations of the black-hole binary GRS1915+105 were
triggered in 2004 (April 17 and 21), during a long "plateau" state of the
source. We analyzed the data collected with EPIC-pn in Timing and Burst modes,
respectively. No thermal disc emission is required by the data; the spectrum is
well fitted by four components: a primary component (either a simple power law
or thermal Comptonization) absorbed by cold matter with abundances different
than those of standard ISM; reprocessing from an ionized disc; emission and
absorption lines; and a soft X-ray excess around 1 keV. The latter is not
confirmed by RGS (which were used in the second observation only); if real, the
excess could be due to reflection from the optically thin, photoionized plasma
of a disc wind, in which case it may provide a way to disentangle intrinsic
from interstellar absorption. Indeed, the former is best traced by the higher
abundances of heavier elements, while an independent estimate of the latter may
be given by the value we get for the disc wind component only, which roughly
coincides with what is found for lower-Z species.
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No Label
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No Label
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astro-ph/0510159
|
David L. Band
|
A Search for Early Optical Emission at Gamma-Ray Burst Locations by the
Solar Mass Ejection Imager (SMEI)
|
astro-ph
|
The Solar Mass Ejection Imager (SMEI) views nearly every point on the sky
once every 102 minutes and can detect point sources as faint as R~10th
magnitude. Therefore, SMEI can detect or provide upper limits for the optical
afterglow from gamma-ray bursts in the tens of minutes after the burst when
different shocked regions may emit optically. Here we provide upper limits for
58 bursts between 2003 February and 2005 April.
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No Label
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No Label
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0803.1509
|
W. Lee Powell Jr.
|
New Findings for the Blue Plume Stars in the Canis Major Over-Density
|
astro-ph
|
We obtained new UBV photometry and spectroscopy of Blue Plume (BP) stars in
fields near the center of the Canis Major Over-Density (CMa). We combined
analysis of the color-color diagrams with a new comparison of the hydrogen
Balmer-line profile to the reddening-free Q parameter to improve the reddening
and extinction estimates for this low-latitude, differentially reddened, area
of the sky. Results of our stellar parameter analysis for B/A spectral type
stars associated with the BP show that the majority of the stars have
main-sequence surface gravities placing them at an average heliocentric
distance of <D> = 6.0 +/- 2.7 kpc. This distance is more consistent with
membership in the intervening Perseus spiral arm and strongly suggests that the
BP stars are not associated with the other stellar populations previously
reported to make up the CMa. This result casts serious doubt on the proposed
dwarf galaxy origin for the CMa.
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No Label
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No Label
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astro-ph/0008283
|
C. Steidel
|
Lyman Continuum Emission from Galaxies at z~3.4
|
astro-ph
|
We report the detection of significant Lyman continuum flux in the composite
spectrum of 29 Lyman break galaxies (LBGs) with redshifts <z> = 3.40+/-0.09.
After correction for opacity due to intervening absorption using a new
composite QSO spectrum evaluated at the same redshift, the ratio of emergent
flux density at 1500 \AA in the rest frame to that in the Lyman continuum is
L(1500)/L(900) = 4.6 +/- 1.0. If the relative intensity of the inferred
escaping Lyman continuum radiation is typical of LBGs at z ~ 3 (the galaxies in
this sample are drawn from the bluest quartile of LBG spectral energy
distributions due to known selection effects), then observed LBGs produce about
5 times more H-ionizing photons per unit co-moving volume than QSOs at z ~ 3.
The associated contribution to the metagalactic ionizing radiation field is
J_{\nu}(912) = 1.2+/-0.3 x 10^{-21} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at z ~
3, very close to most estimates of the radiation field background based on the
``proximity effect''. A preliminary analysis of the density of faint QSOs in
our Lyman break galaxy survey indicates that the standard extrapolated QSO
luminosity function may slightly over-predict the QSO contribution to
J_{\nu}(912) at z ~ 3. We briefly discuss the implications of a
galaxy-dominated UV background at high redshifts.
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No Label
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astro-ph/9812238
|
Lih-Sin The
|
A New Study of s-Process Nucleosynthesis in Massive Stars
|
astro-ph
|
We present a comprehensive study of s-process nucleosynthesis in 15, 20, 25,
and 30 $\msun$ stellar models having solar-like initial composition. The stars
are evolved up to ignition of central neon with a 659 species network coupled
to the stellar models. In this way, the initial composition from one burning
phase to another is consistently determined, especially with respect to neutron
capture reactions. The aim of our calculations is to gain a full account of the
s-process yield from massive stars. In the present work, we focus primarily on
the s-process during central helium burning and illuminate some major
uncertainties affecting the calculations. We briefly show how advanced burning
can significantly affect the products of the core helium burning s-process and,
in particular, can greatly deplete $^{80}$Kr that was strongly overproduced in
the earlier core helium burning phase; however, we leave a complete analysis of
the s-process during the advanced evolutionary phases (especially in shell
carbon burning) to a subsequent paper. Our results can help to constrain the
yield of the s-process material from massive stars during their pre-supernova
evolution.
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No Label
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No Label
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astro-ph/9811352
|
Alan T. Tokunaga
|
K-Band Spectra and Narrowband Photometry of DENIS Field Brown Dwarfs
|
astro-ph
|
Infrared spectra at 1.9-2.5 micron and narrowband photometry of three
low-mass objects, DENIS-P J0205.4-1159, J1058.7-1547, and J1228.2-1547, are
presented. As shown previously by Delfosse et al. (1997, AA, 327, L25), DENIS-P
J0205.4-1159 shows an absorption feature at 2.2 microns. We attribute this
absorption to H_2. A simple two-parameter analysis of the K-band spectrum of
low-mass objects is presented in which the relative strength of the H_2O and
H_2 absorption bands is found to be correlated with the effective temperature
of the objects. The analysis confirms that DENIS-P J0205.4-1159 is the
lowest-temperature object of the three. We present narrow-band photometry of
these objects which provides the continuum flux level inbetween the deep H_2O
absorption bands. These data show the continuum level accurately for the first
time, and they will provide tight constraints for spectral models of these
interesting objects.
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No Label
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No Label
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astro-ph/0603244
|
Eric Gawiser
|
The Physical Nature of Lyman Alpha Emitting Galaxies at z=3.1
|
astro-ph
|
We selected 40 candidate Lyman Alpha Emitting galaxies (LAEs) at z ~=3.1 with
observed frame equivalent widths >150A and inferred emission line fluxes
>2.5x10^-17 ergs/cm^2/s from deep narrow-band and broad-band MUSYC images of
the Extended Chandra Deep Field South. Covering 992 sq. arcmin, this is the
largest ``blank field'' surveyed for LAEs at z ~3, allowing an improved
estimate of the space density of this population of 3+-1x10^-4 h_70^3/Mpc^3.
Spectroscopic follow-up of 23 candidates yielded 18 redshifts, all at z ~=3.1.
Over 80% of the LAEs are dimmer in continuum magnitude than the typical Lyman
break galaxy spectroscopic limit of R= 25.5 (AB), with a median continuum
magnitude R ~=27 and very blue continuum colors, (V-z) ~=0. Over 80% of the
LAEs have the right UVR colors to be selected as Lyman break galaxies, but only
10% also have R<=25.5. Stacking the UBVRIzJK fluxes reveals that LAEs have
stellar masses ~=5x10^8 h_70^-2 M_sun and minimal dust extinction, A_V < ~ 0.1.
Inferred star formation rates are ~=6 h_70^-2 M_sun/yr, yielding a cosmic star
formation rate density of 2x10^-3 h_70 M_sun/yr/Mpc^3. None of our LAE
candidates show evidence for rest-frame emission line equivalent widths
EW_rest>240A which might imply a non-standard IMF. One candidate is detected by
Chandra, implying an AGN fraction of 2+-2% for LAE candidate samples. In
summary, LAEs at z ~ 3 have rapid star formation, low stellar mass, little dust
obscuration and no evidence for a substantial AGN component.
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No Label
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No Label
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astro-ph/0412605
|
Kieran Cleary
|
Source subtraction for the extended Very Small Array and 33-GHz source
count estimates
|
astro-ph
|
We describe the source subtraction strategy and observations for the extended
Very Small Array, a CMB interferometer operating at 33 GHz. A total of 453
sources were monitored at 33 GHz using a dedicated source subtraction baseline.
131 sources brighter than 20 mJy were directly subtracted from the VSA
visibility data. Some characteristics of the subtracted sources, such as
spectra and variability, are discussed. The 33-GHz source counts are estimated
from a sample selected at 15 GHz. The selection of VSA fields in order to avoid
bright sources introduces a bias into the observed counts. This bias is
corrected and the resulting source count is estimated to be complete in the
flux-density range 20-114 mJy. The 33-GHz source counts are used to calculate a
correction to the VSA power spectrum for sources below the subtraction limit.
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No Label
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No Label
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astro-ph/9806366
|
Patrick S. Osmer
|
A Deep Multicolor Survey. IV. The Electronic Stellar Catalog
|
astro-ph
|
We make available in electronic form the stellar catalog of 19,494 objects
from the Deep Multicolor Survey (DMS). The DMS is based on CCD imaging with the
Mayall 4-m telescope in U,B,V,R',I75,I86 and covers 0.83 sq. deg. in six fields
at high galactic latitude. The survey reached 5 sigma limiting magnitudes of
22.1 in I86 to 23.8 in B. The catalog gives positions, magnitudes and error
estimates, and classification codes in the six filter bands for all the
objects. We present tables that summarize the spectroscopic results for the 55
quasars, 44 compact narrow emission-line galaxies, and 135 stars in the DMS
that we have confirmed to date. We also make available illustrations of all the
spectra. The catalog and spectra can be obtained from the World Wide Web at
http://www.astronomy.ohio-state.edu/~posmer/DMS/.
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No Label
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astro-ph/0307235
|
Simone Dall'Osso
|
The glitches of the Anomalous X-ray Pulsar 1RXS J170849.0--400910
|
astro-ph
|
We report on a timing analysis of archival observations of the Anomalous
X-ray Pulsar 1RXSJ170849.0-400910 made with the RXTE Proportional Counter
Array. We detect a new large glitch (Delta nu / nu ~ 3 x 10^-6) which occurred
between 2001 March 27 and 2001 May 6, with an associated large increase in the
spin-down rate (Delta nudot/nudot ~ 0.3). The short time (1.5 yrs) elapsed from
the previously detected glitch and the large amplitude of the new spin-up place
this source among the most frequent glitchers, with large average glitch
amplitudes, similar to those of the Vela pulsar. The source shows different
recoveries after the glitches: in the first one it is well described by a long
term linear trend similar to those seen in Vela-like glitches; in the second
case the recovery is considerably faster and is better described by an
exponential plus a fractional change in the long-term spin-down rate of the
order of 1%. No recovery of the latter is detected but additional observations
are necessary to confirm this result. Observed glitch properties are compared
to those of radio pulsar glitches; current models are discussed in light of our
results. It appears that glitches may represent yet another peculiarity of
AXPs. Starquake-based models appear to be prefered on qualitative grounds.
Alternative models can be applied to individual glitches but fail in explaining
both. Thus the two events may as well arise from two different mechanisms.
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No Label
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astro-ph/0209199
|
Andrew Cumming
|
The Statistics of Extrasolar Planets: Results from the Keck Survey
|
astro-ph
|
We present an analysis of precision radial velocity measurements for 580
stars from the Keck survey. We first discuss the detection threshold of the
survey, and then describe a Bayesian approach to constrain the distribution of
extrasolar planet orbital parameters using both detections and upper limits.
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No Label
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No Label
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astro-ph/9803309
|
Lev Titarchuk
|
The High-Energy Spectra of Accreting Black Holes: Observational Evidence
for Bulk-Motion Infall
|
astro-ph
|
We discuss the emergent spectra from accreting black holes, considering in
particular the case where the accretion is characterized by relativistic bulk
motion. We suggest that such accretion is likely to occur in a wide variety of
black hole enviroments, where the strong gravitational field is expected to
dominate the pressure forces, and this likely to lead to a characteristic
high-energy spectriscopic signature; an extended power tail. It is in the high
(soft) state that matter impinging upon the event horizon can be viewed
directly, and intrinsic power-law seen. A test of the model is presented using
observational data from the Compton Gamma Ray Observatory and the Rossi X-ray
Timing Explorer, covering 2- 200 keV for recent galactic black hole X-ray nova
outbursts.
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No Label
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No Label
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astro-ph/0107208
|
Eva K. Grebel
|
Dwarf Galaxies in the Local Group and in the Local Volume
|
astro-ph
|
After summarizing the characteristics of different types of dwarf galaxies I
briefly review our current state of knowledge of dwarf galaxy evolution in the
Local Group, for which we now have a fairly detailed although by no means
comprehensive picture. All Local Group dwarfs studied to date contain an old
population, though its fraction varies considerably. The majority of the dwarf
companions of the Milky Way shows evidence for a common epoch of ancient star
formation. Spatial variations in star formation are frequently observed in many
dwarf galaxies in the Local Group and beyond. These spatial variations range
from a seemingly stochastic distribution of star-forming regions in gas-rich,
high-mass dwarfs to radial gradients in low-mass dwarfs. The global mode of
star formation may be either continuous with amplitude variations or episodic.
High-mass dwarf galaxies tend to form stars over a Hubble time, whereas
low-mass dwarfs eventually cease to form stars, possibly aided by environmental
effects. Much less is known about the content and properties of dwarf galaxies
in the Local Volume, which we are trying to remedy through a large
observational effort. Dwarf galaxies in the Local Volume follow a similar trend
with absolute magnitude, mean metallicity, and central surface brightness as
the Local Group dwarfs do, and appear to be subject to morphological
segregation.
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No Label
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No Label
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astro-ph/0403625
|
Martin Still
|
Fine-tuning the accretion disk clock in Hercules X-1
|
astro-ph
|
RXTE ASM count rates from the X-ray pulsar Her X-1 began falling consistently
during the late months of 2003. The source is undergoing another state
transition similar to the anomalous low state of 1999. This new event has
triggered observations from both space- and ground-based observatories. In
order to aid data interpretation and telescope scheduling, and to facilitate
the phase-connection of cycles before and after the state transition, we have
re-calculated the precession ephemeris using cycles over the last 3.5 years. We
report that the source has displayed a different precession period since the
last anomalous event. Additional archival data from CGRO suggests that each low
state is accompanied by a change in precession period and that the subsequent
period is correlated with accretion flux. Consequently our analysis reveals
long-term accretion disk behaviour which is predicted by theoretical models of
radiation-driven warping.
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No Label
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No Label
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0804.3946
|
Justus Vogel
|
The serendipituous discovery of a short-period eclipsing polar in 2XMMp
|
astro-ph
|
We report the serendipituous discovery of the new eclipsing polar 2XMMp
J131223.4+173659. Its striking X-ray light curve attracted immediate interest
when we were visually inspecting the source products of the 2XMMp catalogue.
This light curve revealed its likely nature as a magnetic cataclysmic variable
of AM Herculis (or polar) type with an orbital period of ~92 min, which was
confirmed by follow-up optical spectroscopy and photometry. 2XMMp
J131223.4+173659 probably has a one-pole accretion geometry. It joins the group
of now nine objects that show no evidence of a soft component in their X-ray
spectra despite being in a high accretion state, thus escaping ROSAT/EUVE
detection. We discuss the likely accretion scenario, the system parameters, and
the spectral energy distribution.
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No Label
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No Label
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astro-ph/0603844
|
T Jaffe
|
Fast and Efficient Template Fitting of Deterministic Anisotropic
Cosmological Models Applied to WMAP Data
|
astro-ph
|
We explore methods of fitting templates to cosmic microwave background (CMB)
data, and in particular demonstrate the application of the total convolver
algorithm as a fast method of performing a search over all possible locations
and orientations of the template relative to the sky. This analysis includes
investigation of issues such as chance alignments and foreground residuals. We
apply these methods to compare Bianchi models of type VII_h to WMAP first year
data and confirm the basic result of our 2005 paper.
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No Label
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No Label
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astro-ph/0506157
|
Shiho Kobayashi
|
Inverse Compton X-ray Flare from GRB Reverse Shock
|
astro-ph
|
We study synchrotron self-inverse Compton radiation from a reverse-shocked
fireball. If the inverse Compton process dominates the cooling of shocked
electrons, an X-ray flare produced by the first order Compton scattering would
emerge in the very early afterglow phase, with the bulk of the shock energy
radiated in the second order scattering component at 10-100 MeV. The dominance
of inverse Compton cooling leads to the lack of prompt optical flashes. We show
that for plausible parameters this scattering process can produce an X-ray
flare with a relative amplitude change by a factor of several. Flares with a
larger amplitude and multiple X-ray flares in a single event are likely to be
produced by another mechanism (e.g. internal shocks).
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No Label
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No Label
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astro-ph/0203445
|
Giuseppe Longo
|
Neural Networks and Photometric Redshifts
|
astro-ph
|
We present a neural network based approach to the determination of
photometric redshift. The method was tested on the Sloan Digital Sky Survey
Early Data Release (SDSS-EDR) reaching an accuracy comparable and, in some
cases, better than SED template fitting techniques. Different neural networks
architecture have been tested and the combination of a Multi Layer Perceptron
with 1 hidden layer (22 neurons) operated in a Bayesian framework, with a Self
Organizing Map used to estimate the accuracy of the results, turned out to be
the most effective. In the best experiment, the implemented network reached an
accuracy of 0.020 (interquartile error) in the range 0<zphot<0.3, and of 0.022
in the range 0<zphot<0.5.
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No Label
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No Label
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astro-ph/9811160
|
Bianca Garilli
|
Composite luminosity function of cluster galaxies
|
astro-ph
|
We constructed the composite Luminosity Function (LF) of cluster galaxies in
the g,r and i bands from the photometry of a mixed (Abell and X-ray selected)
sample of the cores of 65 clusters, ranging in redshift from 0.05 to 0.25. The
composite LF has been obtained from complete samples of ~2200 galaxies in the
magnitude range -23<M<-17.5 (-18 in i). Cluster membership has been determined
on the basis of color-color plots for each cluster and the resulting outlier
counts have been checked against field counts in the same bands. We find that
the galaxy density of the environment determines the shape of the LF, in the
sense that bright galaxies are more abundant in dense clusters.
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No Label
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No Label
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astro-ph/9609034
|
Charley Lineweaver
|
The CMB Dipole: The Most Recent Measurement And Some History
|
astro-ph
|
The largest anisotropy in the cosmic microwave background (CMB) is the
$\approx 3$ mK dipole assumed to be due to our velocity with respect to the
CMB. Over the past ten years the precision of our knowledge of the dipole has
increased by a factor of ten. We discuss the most recent measurement of this
dipole obtained from the four year COBE Differential Microwave Radiometers
(DMR) as reported by Lineweaver \etal (1996). The inferred velocity of the
Local Group is $v_{LG}= 627 \pm 22$ km/s in the direction $\ell = 276\deg \pm
3$, $b= 30\deg \pm 2$. We compare this most recent measurement to a compilation
of more than 30 years of dipole observations.
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No Label
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No Label
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astro-ph/0511268
|
Ivan Agudo
|
Present and Future Millimeter VLBI Imaging of Jets in AGN: The Case of
NRAO 150
|
astro-ph
|
The Global mm-VLBI Array is at present the most sensitive 3 mm-VLBI
interferometer and provides images of up to 40 micro-arcsecond resolution.
Using this array, we have monitored the rotation of the innermost jet in the
quasar NRAO 150, which shows an angular speed of ~ 7 deg./yr. Future 3 mm
arrays could include additional stations like ALMA, GBT, LMT, CARMA, SRT,
Yebes, Nobeyama and Noto, which would allow to push VLBI at this wavelength to
sensitivity and image quality levels comparable to those of present VLBI at
centimeter wavelengths. This would improve our knowledge of the accretion
systems and the magneto-hydrodynamics of the innermost jets in AGN and
microquasars.
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No Label
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No Label
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astro-ph/9804112
|
Andrew Lobanov
|
Spectral distributions in compact radio sources I. Imaging with VLBI
data
|
astro-ph
|
We discuss a technique for mapping the synchrotron turnover frequency
distribution using nearly simultaneous, multi--frequency VLBI observations. The
limitations of the technique arising from limited spatial sampling and
frequency coverage are investigated. The errors caused by uneven spatial
sampling of typical multi--frequency VLBA datasets are estimated through
numerical simulations, and are shown to be of the order of 10%, for pixels with
the deconvolution SNR~7. The fitted spectral parameters are corrected for the
errors due to limited frequency coverage of VLBI data. First results from
mapping the turnover frequency distribution in 3C345 are presented.
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No Label
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No Label
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0705.1206
|
Jan-Uwe Ness
|
The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.:
Data and preliminary Modeling
|
astro-ph
|
The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst
of the recurrent nova RS Oph was observed twice with Chandra and once with
XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after
outburst. We confirm a 35-sec period on day 54.0 and found that it originates
from the SSS emission and not from the shock. We discus the bound-free
absorption by neutral elements in the line of sight, resonance absorption lines
plus self-absorbed emission line components, collisionally excited emission
lines from the shock, He-like intersystem lines, and spectral changes during an
episode of high-amplitude variability. We find a decrease of the oxygen K-shell
absorption edge that can be explained by photoionization of oxygen. The
absorption component has average velocities of -1286+-267 km/s on day 39.7 and
of -771+-65 km/s on day 66.9. The wavelengths of the emission line components
are consistent with their rest wavelengths as confirmed by measurements of
non-self absorbed He-like intersystem lines. We have evidence that these lines
originate from the shock rather than the outer layers of the outflow and may be
photoexcited in addition to collisional excitations. We found collisionally
excited emission lines that are fading at wavelengths shorter than 15A that
originate from the radiatively cooling shock. On day 39.5 we find a systematic
blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i
ratios which indicates either high densities or significant UV radiation near
the plasma where the emission lines are formed. During the phase of strong
variability the spectral hardness light curve overlies the total light curve
when shifted by 1000sec. This can be explained by photoionization of neutral
oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.
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No Label
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astro-ph/0610381
|
Jes K. J{\o}rgensen
|
Current Star Formation in the Perseus Molecular Cloud: Constraints from
Unbiased Submillimeter and Mid-Infrared Surveys
|
astro-ph
|
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Perseus molecular cloud complex based on a combination of
Spitzer Space Telescope mid-IR data from the c2d legacy team and JCMT/SCUBA
submillimeter maps from the COMPLETE team. The mid-IR sources detected at 24
micron and having [3.6]-[4.5] > 1 are located close to the center of the SCUBA
cores, typically within 15" of their peaks. The narrowness of the spatial
distribution of mid-IR sources around the peaks of the SCUBA cores suggests
that no significant dispersal of the newly formed YSOs has occurred. This
argues against the suggestion that motions of protostars regulate the time
scales over which significant (Bondi-Hoyle) accretion can occur. The most
deeply embedded YSOs are found in regions with high extinction, AV > 5, similar
to the extinction threshold observed for the SCUBA cores. All the SCUBA cores
with high concentrations have embedded YSOs, but not all cores with low
concentrations are starless. An unbiased sample of 49 deeply embedded YSOs is
constructed. Embedded YSOs are found in 40 of the 72 SCUBA cores with only
three cores harboring multiple embedded YSOs within 15". The equal number of
SCUBA cores with and without embedded YSOs suggests that the time scale for the
evolution through the dense prestellar stages, where the cores are recognized
in the submillimeter maps and have central densities of 5e4-1e5 cm^{-3}, is
similar to the time scale for the embedded protostellar stages. The current
star formation efficiency of cores is estimated to be approximately 10-15%. In
contrast, the star formation efficiency averaged over the cloud life time and
compared to the total cloud mass is only a few percent, reflecting also the
efficiency in assembling cloud material into the dense cores forming stars.
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No Label
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No Label
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0810.1141
|
Martin Durant
|
Multiwavelength spectral and high time resolution observations of SWIFT
J1753.5-0127: new activity?
|
astro-ph
|
We have conducted an extensive observational campaign of SWIFT J1753.5-0127
during June 2007 after its bright outburst episode in 2005. We have performed
multi-band optical photometry, optical spectroscopy, X-ray spectroscopy and
timing and ULTRACAM optical photometry simultaneously in three bands. Both the
optical spectrum and the X-ray spectrum, along with enhanced brightness in
broad-band photometry point to recent increased activity. We analyze the
different spectral regions, finding a smooth optical continuum with a
remarkable lack of lines and a very blue component modulated with a period of
3.2hr and a hard power-law X-ray spectrum. Both the X-ray and optical power
spectra are flat at low frequencies up to the 0.1 Hz (10 s) range, then
decreasing roughly as a power law consistent with flickering. Furthermore, the
optical data show quasi-periodic oscillations (QPOs) near 0.08 Hz (13 s).
Together with a dynamical and auto-correlation analysis of the light curves we
attempt to construct a complete physical picture of this intriguing system.
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No Label
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No Label
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0708.3083
|
David Merritt
|
Dynamics around supermassive black holes
|
astro-ph
|
The dynamics of galactic nuclei reflects the presence of supermassive black
holes (SBHs) in many ways. Single SBHs act as sinks, destroying a mass in stars
equal to their own mass in roughly one relaxation time and forcing nuclei to
expand. Formation of binary SBHs displaces a mass in stars roughly equal to the
binary mass, creating low-density cores and ejecting hyper-velocity stars.
Gravitational radiation recoil can eject coalescing binary SBHs from nuclei,
resulting in offset SBHs and lopsided cores. We review recent work on these
mechanisms and discuss the observable consequences.
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No Label
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No Label
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astro-ph/9910402
|
Eva K. Grebel
|
The dwarf spheroidal galaxy DDO 44: stellar populations and distance
|
astro-ph
|
We present ground-based and HST WFPC2 imaging of the nearby low surface
brightness dwarf spheroidal galaxy DDO 44. For the first time DDO 44 was
resolved into stars. The resulting color-magnitude diagram for about 1290 stars
shows the red giant branch with a tip at I=(23.55+-0.15) mag, which yields a
distance of (3.2+-0.2) Mpc consistent with membership of DDO 44 in the NGC 2403
group. The linear separation of DDO 44 from NGC 2403 is 75 kpc on the sky and
(30+-450) kpc along the line of sight. The relationship between the dwarf
galaxy's absolute R magnitude of -13.1 mag, its central surface brightness of
24.1 mag/square arcsec, and its mean metallicity of -1.7 dex follow the trend
for other nearby dwarf spheroidal galaxies. One globular cluster candidate has
also been identified in DDO 44.
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No Label
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astro-ph/0012135
| null |
Helium contamination from the progenitor stars of planetary nebulae: The
He/H radial gradient and the Delta Y/Delta Z enrichment ratio
|
astro-ph
|
In this work, two aspects of the chemical evolution of 4He in the Galaxy are
considered on the basis of a sample of disk planetary nebulae (PN). First, an
application of corrections owing to the contamination of 4He from the evolution
of the progenitor stars shows that the He/H abundance by number of atoms is
reduced by 0.012 to 0.015 in average, leading to an essentially flat He/H
radial distribution. Second, a determination of the helium to heavy element
enrichment ratio using the same corrections leads to values in the range 2.8 <
Delta Y/Delta Z < 3.6 for Yp = 0.23 and 2.0 < Delta Y/Delta Z < 2.8 for Yp =
0.24, in good agreement with recent independent determinations and theoretical
models.
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No Label
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No Label
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0710.3934
|
Igor Rogachevskii
|
New mechanism of generation of large-scale magnetic field in a sheared
turbulent plasma
|
astro-ph
|
A review of recent studies on a new mechanism of generation of large-scale
magnetic field in a sheared turbulent plasma is presented. This mechanism is
associated with the shear-current effect which is related to the W x J-term in
the mean electromotive force. This effect causes the generation of the
large-scale magnetic field even in a nonrotating and nonhelical homogeneous
sheared turbulent convection whereby the alpha effect vanishes. It is found
that turbulent convection promotes the shear-current dynamo instability, i.e.,
the heat flux causes positive contribution to the shear-current effect.
However, there is no dynamo action due to the shear-current effect for small
hydrodynamic and magnetic Reynolds numbers even in a turbulent convection, if
the spatial scaling for the turbulent correlation time is k^{-2}, where k is
the small-scale wave number. We discuss here also the nonlinear mean-field
dynamo due to the shear-current effect and take into account the transport of
magnetic helicity as a dynamical nonlinearity. The magnetic helicity flux
strongly affects the magnetic field dynamics in the nonlinear stage of the
dynamo action. When the magnetic helicity flux is not small, the saturated
level of the mean magnetic field is of the order of the equipartition field
determined by the turbulent kinetic energy. The obtained results are important
for elucidation of origin of the large-scale magnetic fields in astrophysical
and cosmic sheared turbulent plasma.
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No Label
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No Label
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astro-ph/0612622
|
Gregor Rauw
|
Early-type stars in the core of the young open cluster Westerlund2
|
astro-ph
|
Aims. The properties of the early-type stars in the core of the Westerlund2
cluster are examined in order to establish a link between the cluster and the
very massive Wolf-Rayet binary WR20a as well as the HII complex RCW49.
Methods. Photometric monitoring as well as spectroscopic observations of
Westerlund2 are used to search for light variability and to establish the
spectral types of the early-type stars in the cluster core.
Results. The first light curves of the eclipsing binary WR20a in B and V
filters are analysed and a distance of 8kpc is inferred. Three additional
eclipsing binaries, which are probable late O or early B-type cluster members,
are discovered, but none of the known early O-type stars in the cluster
displays significant photometric variability above 1% at the 1-sigma level. The
twelve brightest O-type stars are found to have spectral types between O3 and
O6.5, significantly earlier than previously thought.
Conclusions. The distance of the early-type stars in Westerlund2 is
established to be in excellent agreement with the distance of WR20a, indicating
that WR20a actually belongs to the cluster. Our best estimate of the cluster
distance thus amounts to 8.0pm1.4kpc. Despite the earlier spectral types, the
currently known population of early-type stars in Westerlund2 does not provide
enough ionizing photons to account for the radio emission of the RCW49 complex.
This suggests that there might still exist a number of embedded early O-stars
in RCW49.
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No Label
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No Label
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astro-ph/9511063
|
Chris Reynolds
|
The variable OVIII Warm Absorber in MCG-6-30-15
|
astro-ph
|
We present the results of a 4 day ASCA observation of the Seyfert galaxy
MCG-6-30-15, focussing on the nature of the X-ray absorption by the warm
absorber, characterizd by the K-edges of the intermediately ionized oxygen,
OVII and OVIII. We confirm that the column density of OVIII changes on a
timescale of $\sim 10^4$~s when the X-ray continuum flux decreases. The
significant anti-correlation of column density with continuum flux gives direct
evidence that the warm absorber is photoionized by the X-ray continuum. From
the timescale of the variation of the OVIII column density, we estimate that it
originates from gas within a radius of about $10^{17}\cm$ of the central
engine. In contrast, the depth of the OVII edge shows no response to the
continuum flux, which indicates that it originates in gas at larger radii. Our
results strongly suggest that there are two warm absorbing regions; one located
near or within the Broad Line Region, the other associated with the outer
molecular torus, scattering medium or Narrow Line Region.
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No Label
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No Label
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astro-ph/9611110
|
Hideki Asada
|
Critical Lines in Gravitational Lenses and the Determination of
Cosmological Parameters
|
astro-ph
|
We investigate the cosmological test recently proposed by B. Fort, Y. Mellier
and M. Dantel-Fort (FMD), where the observed location of the critical line in
gravitational lensing is used to determine the cosmological parameters,
$\Omega$ and $\lambda$. Applying this method to the cluster of galaxies
Cl0024+1654, FMD obtained a constraint on the cosmological constant, $\lambda >
0.6$, assuming the spatially flat universe. It plays a crucial role in this
method that the angular diameter distance-redshift relation depends on the
cosmological models through the cosmological parameters. First, using the
angular diameter distance in the Friedmann-Lemaitre- Robertson-Walker universe,
we show that one can hardly determine $\Omega$ by this method without the
assumption of the spatially flat universe. We also investigate the effect of
inhomogeneities of the universe by using the Dyer-Roeder angular diameter
distance. It is shown that the effect of inhomogeneities can become too large
to be ignored, particularly for a high density universe. As a result, this
method cannot be taken as a clear cosmological test to determine $\Omega$ and
$\lambda$, though it may provide a bound on $\Omega$ and $\lambda$. Moreover,
we mention the uncertainty of the determination of the velocity dispersion,
which is regarded as one of the most serious problems in this test.
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No Label
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0804.0839
|
Ronald Mennickent
|
The eclipsing LMC star OGLE05155332--6925581: a clue for Double Periodic
Variables
|
astro-ph
|
We investigate the nature of OGLE05155332-6925581, one of the brightest
members of the enigmatic group of Double Periodic Variables (DPVs) recently
found in the Magellanic Clouds. The modeling of archival orbital light curves
(LCs), along with the analysis of the radial velocities suggest that this
object is a semi--detached binary with the less massive star transferring
matter to the more massive and less evolved star, in an Algol--like
configuration. We find evidence for additional orbital variability and
H$\alpha$ emission, likely caused by an accretion disc around the primary star.
As in the case of $\beta Lyr$ the circumprimary disc seems to be more luminous
than the primary, but we do not detect orbital period changes. We find that the
LC follows a loop in the color--magnitude diagram during the long cycle; the
system is redder when brighter and the rising phase is bluer than during
decline. Infrared excess is also present. The source of the long--term
periodicity is not eclipsed, indicating its circumbinary origin. Strong
asymmetries, discrete absorption components (DACs) and a $\gamma$ shift are new
and essential observational properties in the infrared H I lines. The DACs
strength and RV follow a saw--teeth pattern during the orbital cycle. We
suggest that the system experiences supercycles of mass outflow feeding a
circumbinary disc. Mass exchange and mass loss could produce comparable but
opposite effects in the orbital period on a long time scale, resulting in a
quasi--constancy of this parameter.
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No Label
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No Label
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astro-ph/0612256
|
Lijun Gou
|
Modeling GRB 050904: Autopsy of a Massive Stellar Explosion at z=6.29
|
astro-ph
|
GRB 050904 at redshift z=6.29, discovered and observed by Swift and with
spectroscopic redshift from the Subaru telescope, is the first gamma-ray burst
to be identified from beyond the epoch of reionization. Since the progenitors
of long gamma-ray bursts have been identified as massive stars, this event
offers a unique opportunity to investigate star formation environments at this
epoch. Apart from its record redshift, the burst is remarkable in two respects:
first, it exhibits fast-evolving X-ray and optical flares that peak
simultaneously at t~470 s in the observer frame, and may thus originate in the
same emission region; and second, its afterglow exhibits an accelerated decay
in the near-infrared (NIR) from t~10^4 s to t~3 10^4 s after the burst,
coincident with repeated and energetic X-ray flaring activity. We make a
complete analysis of available X-ray, NIR, and radio observations, utilizing
afterglow models that incorporate a range of physical effects not previously
considered for this or any other GRB afterglow, and quantifying our model
uncertainties in detail via Markov Chain Monte Carlo analysis. In the process,
we explore the possibility that the early optical and X-ray flare is due to
synchrotron and inverse Compton emission from the reverse shock regions of the
outflow. We suggest that the period of accelerated decay in the NIR may be due
to suppression of synchrotron radiation by inverse Compton interaction of X-ray
flare photons with electrons in the forward shock; a subsequent interval of
slow decay would then be due to a progressive decline in this suppression. The
range of acceptable models demonstrates that the kinetic energy and circumburst
density of GRB 050904 are well above the typical values found for low-redshift
GRBs.
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astro-ph/0208016
|
Mustapha Mouhcine
|
Modelling of intermediate-age stellar populations: III Effects of
dust-shells around AGB stars
|
astro-ph
|
In this paper,we present single stellar population models of intermediate age
stellar populations where dust-enshrouded AGB stars are introduced. The
formation of carbon stars is also accounted for, and is taken to be a function
of both initial mass and metallicity. The effect of the dusty envelopes around
AGB stars on the optical/near-infrared spectral energy distribution were
introduced using semi-emipirical models where the mass-loss and the
photospheric chemistry determine the spectral properties of a star along the
AGB sequence. The spectral dichotomy between O-rich stars and C-rich stars is
taken into account in the modelling. We have investigated the AGB sequence
morphology in he near-infrared CMD as a function of time and metallicity. We
show that this diaggram is characterized by three morphological features,
occupied by optically bright O-rich stars, optically bright C-rich stars, and
dust-enshrouded O-rich and C-rich stars respectively. Our models are able to
reproduce the distribution of the three AGB subtype stellar populations in
colour-colour diagrams. Effects of dusty envelopes on the luminosity function
are also investigated (Abriged).
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astro-ph/0210494
|
Lawrence M. Krauss
|
Strong lensing constraints on the velocity dispersion and density
profile of elliptical galaxies
|
astro-ph
|
We use the statistics of strong gravitational lensing from the CLASS survey
to impose constraints on the velocity dispersion and density profile of
elliptical galaxies. This approach differs from much recent work, where the
luminosity function, velocity dispersion and density profile were typically
{\it assumed} in order to constrain cosmological parameters. It is indeed
remarkable that observational cosmology has reached the point where we can
consider using cosmology to constrain astrophysics, rather than vice versa. We
use two different observables to obtain our constraints (total optical depth
and angular distributions of lensing events). In spite of the relatively poor
statistics and the uncertain identification of lenses in the survey, we obtain
interesting constraints on the velocity dispersion and density profiles of
elliptical galaxies. For example, assuming the SIS density profile and
marginalizing over other relevant parameters, we find 168 km/s < sigma_* < 200
km/s (68% CL), and 158 km/s < sigma_* < 220 km/s (95% CL). Furthermore, if we
instead assume a generalized NFW density profile and marginalize over other
parameters, the slope of the profile is constrained to be 1.50 < beta < 2.00
(95% CL). We also constrain the concentration parameter as a function of the
density profile slope in these models. These results are essentially
independent of the exact knowledge of cosmology. We briefly discuss the
possible impact on these constraints of allowing the galaxy luminosity function
to evolve with redshift, and also possible useful future directions for
exploration.
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No Label
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astro-ph/0507119
|
Troy Porter
|
A new estimate of the Galactic interstellar radiation field between 0.1
microns and 1000 microns
|
astro-ph
|
Cosmic-ray electrons and positrons propagating in the Galaxy produce diffuse
gamma-rays via the inverse Compton (IC) process. The low energy target photon
populations with which the cosmic-rays interact during propagation are produced
by stars, this stellar light being reprocessed by Galactic dust. Detailed
modelling of the Galactic stellar distribution, dust distribution, and
treatment of the absorption and scattering of light is therefore required to
obtain accurate models for the low energy Galactic photon distribution and
spectrum. Using a realistic Galactic stellar distribution model, and dust
distribution, we calculate the diffuse radiation field from stars in the Galaxy
(the `optical' radiation field), including absorption and scattering. Using a
dust heating code, we self-consistently calculate the infra-red radiation field
for the same dust model used for the optical calculation; both transient and
equilibrium heating are included. We present the calculated radiation field
spectra and distributions, and will use these to calculate the expected
Galactic diffuse IC gamma-ray spectrum.
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No Label
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No Label
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0806.4184
|
Carmelo Evoli
|
The puzzling origin of the 6Li plateau
|
astro-ph
|
We discuss the 6Li abundance evolution within a hierarchical model of Galaxy
formation which correctly reproduces the [Fe/H] distribution of metal-poor halo
stars. Contrary to previous findings, we find that neither the level
(6Li/H=6x10^-12) nor the flatness of the 6Li distribution with [Fe/H] can be
reproduced under the most favourable conditions by any model in which 6Li
production is tied to a (data-constrained) Galactic star formation rate via
cosmic ray spallation. Thus, the origin of the plateau might be due to some
other early mechanism unrelated to star formation.
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No Label
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No Label
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0812.1797
|
Konstantinos Tzirakis
|
Tensors, non-Gaussianities, and the future of potential reconstruction
|
astro-ph
|
We present projections for reconstruction of the inflationary potential
expected from ESA's upcoming Planck Surveyor CMB mission. We focus on the
effects that tensor perturbations and the presence of non-Gaussianities have on
reconstruction efforts in the context of non-canonical inflation models. We
consider potential constraints for different combinations of
detection/null-detection of tensors and non-Gaussianities. We perform Markov
Chain Monte Carlo and flow analyses on a simulated Planck-precision data set to
obtain constraints. We find that a failure to detect non-Gaussianities
precludes a successful inversion of the primordial power spectrum, greatly
affecting uncertainties, even in the presence of a tensor detection. In the
absence of a tensor detection, while unable to determine the energy scale of
inflation, an observable level of non-Gaussianities provides correlations
between the errors of the potential parameters, suggesting that constraints
might be improved for suitable combinations of parameters. Constraints are
optimized for a positive detection of both tensors and non-Gaussianities.
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0802.2524
|
Sergei Zharkov Dr
|
Time Distance Study of Isolated Sunspots
|
astro-ph
|
We present a comparative seismic study of conditions around and beneath
isolated sunspots. Using the European Grid of Solar Observations' Solar Feature
Catalogue of sunspots derived from SOHO/MDI continuum and magnetogram data,
1996-2005, we identify a set of isolated sunspots by checking that within a
Carrington Rotation there were no other spots detected in the vicinity. We then
use level-2 tracked MDI Dopplergrams available from SOHO website to investigate
wave-speed perturbations of such sunspots using time-distance helioseismology.
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No Label
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No Label
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astro-ph/0411312
|
Anne Rajala
|
Photometric Typing Analyses of Three Young Supernovae with the Robotic
Palomar 60-Inch Telescope
|
astro-ph
|
We present photometric typing analyses of three young supernovae observed
with the Robotic 60-inch Telescope at Palomar Observatory (P60). This
represents the first time that such photo-typing has been attempted, in a blind
fashion, on newly-discovered supernovae. For one of the target supernovae,
SN2004cs, our photometry provided the first constraint on the SN type, which we
predicted would be type Ia. To the contrary, however, our subsequent Keck
spectroscopy shows it to be an unusual type II supernova. For each of the other
two supernovae, SN2004dh (type II) and SN2004dk (type Ib), our photo-typing
results are consistent with the known type of the event as determined from
ground-based spectroscopy; however, the colors of SN2004dk are also consistent
with a type Ic or type II classification. We discuss our approach to the
challenges of photo-typing - contamination by host galaxy light and the unknown
photometric quality of the data - in the case where it is desirable to complete
the analysis with just one night of observations. The growing interest in the
properties and behavior of very young supernovae, and the increased discovery
rate for such events, mean that prompt photo-typing analyses can provide useful
input to observational campaigns. Our results demonstrate the value and
feasibility of such a project for P60, at the same time illustrating its chief
inherent shortcoming: an inability to identify new and unusual events as such
without later spectroscopic observations.
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astro-ph/0703453
|
Geoffrey Clayton
|
Very Large Excesses of 18O in Hydrogen-Deficient Carbon and R Coronae
Borealis Stars: Evidence for White Dwarf Mergers
|
astro-ph
|
We have found that at least seven hydrogen-deficient carbon (HdC) and R
Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases
less than unity, values that are orders of magnitude lower than measured in
other stars (the Solar value is 500). Greatly enhanced 18O is evident in every
HdC and RCB we have measured that is cool enough to have detectable CO bands.
The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB
stars. These discoveries are important clues in determining the evolutionary
pathways of HdC and RCB stars, for which two models have been proposed: the
double degenerate (white dwarf (WD) merger), and the final helium-shell flash
(FF). No overproduction of 18O is expected in the FF scenario. We have
quantitatively explored the idea that HdC and RCB stars originate in the
mergers of CO- and He-WDs. The merger process is estimated to take only a few
days, with accretion rates of 150 Msun/ yr producing temperatures at the base
of the accreted envelope of 1.2 - 1.9 x 10^8 K. Analysis of a simplified
one-zone calculation shows that nucleosynthesis in the dynamically accreting
material may provide a suitable environment for a significant production of
18O, leading to very low values of 16O/18O, similar to those observed. We also
find qualitative agreement with observed values of 12C/13C and with the CNO
elemental ratios. H-admixture during the accretion process from the small
H-rich C/O WD envelope may play an important role in producing the observed
abundances. Overall our analysis shows that WD mergers may very well be the
progenitors of O18-rich RCB and HdC stars, and that more detailed simulations
and modeling are justified.
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0709.1686
|
Svetozar Zhekov
|
Colliding Stellar Wind Models with Nonequilibrium Ionization: X-rays
from WR 147
|
astro-ph
|
The effects of nonequilibrium ionization are explicitly taken into account in
a numerical model which describes colliding stellar winds (CSW) in massive
binary sytems. This new model is used to analyze the most recent X-ray spectra
of the WR+OB binary system WR 147. The basic result is that it can adequately
reproduce the observed X-ray emission (spectral shape, observed flux) but some
adjustment in the stellar wind parameters is required. Namely, (i) the stellar
wind velocities must be higher by a factor of 1.4 - 1.6; (ii) the mass loss
must be reduced by a factor of ~ 2. The reduction factor for the mass loss is
well within the uncertainties for this parameter in massive stars, but given
the fact that the orbital parameters (e.g., inclination angle and eccentricity)
are not well constrained for WR 147, even smaller corrections to the mass loss
might be sufficient. Only CSW models with nonequilibrium ionization and equal
(or nearly equal) electron and ion postshock temperature are successful.
Therefore, the analysis of the X-ray spectra of WR 147 provides evidence that
the CSW shocks in this object must be collisionless.
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astro-ph/0507578
|
Catrina M. Hamilton Ph.D.
|
The Disappearing Act of KH 15D: Photometric Results from 1995 to 2004
|
astro-ph
|
We present results from the most recent (2002-2004) observing campaigns of
the eclipsing system KH 15D, in addition to re-reduced data obtained at Van
Vleck Observatory (VVO) between 1995 and 2000. Phasing nine years of
photometric data shows substantial evolution in the width and depth of the
eclipses. The most recent data indicate that the eclipses are now approximately
24 days in length, or half the orbital period. These results are interpreted
and discussed in the context of the recent models for this system put forward
by Winn et al. and Chiang & Murray-Clay. A periodogram of the entire data set
yields a highly significant peak at 48.37 +/- 0.01 days, which is in accord
with the spectroscopic period of 48.38 +/- 0.01 days determined by Johnson et
al. Another significant peak, at 9.6 days, was found in the periodogram of the
out-of-eclipse data at two different epochs. We interpret this as the rotation
period of the visible star and argue that it may be tidally locked in
pseudosynchronism with its orbital motion. If so, application of Hut's theory
implies that the eccentricity of the orbit is e = 0.65 +/- 0.01. Analysis of
the UVES/VLT spectra obtained by Hamilton et al. shows that the v sin(i) of the
visible star in this system is 6.9 +/- 0.3 km/sec. Using this value of v sin(i)
and the measured rotation period of the star, we calculate the lower limit on
the radius to be R = (1.3 +/- 0.1), R_Sun, which concurs with the value
obtained by Hamilton et al. from its luminosity and effective temperature. Here
we assume that i = 90 degrees since it is likely that the spin and orbital
angular momenta vectors are nearly aligned.
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astro-ph/9312011
| null |
Dark Matter Annihilations in the Large Magellanic Cloud
|
astro-ph
|
The flat rotation curve obtained for the outer star clusters of the Large
Magellanic Cloud is suggestive of an LMC dark matter halo. From the composite
HI and star cluster rotation curve, I estimate the parameters of an isothermal
dark matter halo added to a `maximum disk.' I then examine the possibility of
detecting high energy gamma-rays from non-baryonic dark matter annihilations in
the central region of the Large Magellanic Cloud.
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No Label
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No Label
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astro-ph/9903405
|
Cecilia
|
Modeling supernova emission at late times
|
astro-ph
|
We compare model calculations with observations of supernovae at late times
to infer the time evolution of temperature, ionization and line emission. Here
we mainly report on our results from our modeling of SN 1987A. We discuss the
oxygen mass from the modeling of line fluxes. Line profiles show the
distribution of the elements and the importance of including time dependence in
the calculations. We discuss different approaches to determine the 44Ti-mass.
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No Label
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No Label
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astro-ph/0701505
|
Simon Pustilnik
|
HI study of extremely metal-deficient dwarf galaxies. I. The Nancay
Radio Telescope observations of twenty-two objects
|
astro-ph
|
The goal of this study is to measure parameters of the integrated HI emission
for twenty-two dwarf galaxies with oxygen abundance 12+log(O/H) in the range of
7.42 to 7.65, which are representatives of the eXtremely Metal-Deficient (XMD)
galaxy group. Some of them are expected to be similar to the well-known
candidates for local young galaxies, IZw18 and SBS 0335-052 that have most of
their baryon mass in the form of neutral gas. Therefore, the HI 21-cm line
observations are crucial to understanding their group and individual
properties. The Nancay Radio Telescope (NRT) with the upgraded focal receiver
was used for observations of the 21-cm HI-line. This permitted the detection of
the faintest sources with rms of ~1 mJy per 10.5 km/s resolution element. For
eighteen detected galaxies we present the parameters of their integrated HI
line emission and describe the data on individual objects in more detail. For
four undetected XMD galaxies, we give upper limits on their M$(HI). For 70% of
the twenty studied non low surface brightness XMD galaxies, we find evidence
(both from HI and optical data) for their interaction with neighboring objects.
In the brief discussion of the group HI properties of the observed subsample
(the total O/H range is of 0.23 dex, or a factor of 1.7), we underline the
broad distributions of the HI mass (range is of 2 orders of magnitude), of the
ratio M(HI)/L_B (of 1 order of magnitude), and of the blue luminosity (range is
of 2 orders of magnitude). We also obtained HI parameters of six galaxies that
do not belong to the XMD sample. These data increase the number of XMD galaxies
with known integrated HI parameters (or upper limits) by a factor of two. This
allows us to address statistical properties of this group, which will be
presented in a forthcoming paper. (Abridged).
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astro-ph/0309531
|
Valery V. Kravtsov
|
Self-Induced Formaton of Metal-Rich Globulars in Bulges?
|
astro-ph
|
Taken together, key latest observations assume that (i) old metal-rich
globular cluster populations (MRGCPs) in bulges were able to form due to
essentially increased, self(internally)-induced star formation rate (SFR) in
the hosts, while galaxy merging played an additional role; (ii) massive star
cluster populations (MSCPs) in irregulars may be young, less prominent
counterparts of the old MRGCPs in spheroids.
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No Label
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astro-ph/0208354
|
William A. Heindl
|
Chandra High Resolution Camera Imaging of GRS 1758-258
|
astro-ph
|
We observed the "micro-quasar" GRS 1758-258 four times with Chandra. Two
HRC-I observations were made in 2000 September-October spanning an
intermediate-to-hard spectral transition (identified with RXTE). Another HRC-I
and an ACIS/HETG observation were made in 2001 March following a hard-to-soft
transition to a very low flux state. Based on the three HRC images and the HETG
zero order image, the accurate position (J2000) of the X-ray source is RA = 18h
01m 12.39s, Dec = -25d 44m 36.1s (90% confidence radius = 0".45), consistent
with the purported variable radio counterpart. All three HRC images are
consistent with GRS 1758-258 being a point source, indicating that any bright
jet is less than ~1 light-month in projected length, assuming a distance of 8.5
kpc.
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No Label
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No Label
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0811.3918
|
Matias Zaldarriaga
|
CMBPol Mission Concept Study: Reionization Science with the Cosmic
Microwave Background
|
astro-ph
|
We summarize existing constraints on the epoch of reionization and discuss
the observational probes that are sensitive to the process. We focus on the
role large scale polarization can play. Polarization probes the integrated
optical depth across the entire epoch of reionization. Future missions such as
Planck and CMBPol will greatly enhance our knowledge of the reionization
history, allowing us to measure the time evolution of the ionization fraction.
As large scale polarization probes high redshift activity, it can best
constrain models where the Universe was fully or partially ionized at early
times. In fact, large scale polarization could be our only probe of the highest
redshifts.
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No Label
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No Label
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astro-ph/0505412
|
Bugaev
|
High Energy Neutrinos as a Probe for New Physics and Astrophysics
|
astro-ph
|
A review of the recent achievements in high energy neutrino physics and,
partly, neutrino astrophysics is presented. It is argued that experiments with
high energy neutrinos of natural origin can be used for a search of new physics
effects beyond the electroweak scale.
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