id
stringlengths 9
16
| submitter
stringlengths 2
51
⌀ | title
stringlengths 5
243
| categories
stringlengths 5
69
| abstract
stringlengths 23
3.66k
| labels
stringlengths 5
184
| domain
stringclasses 9
values |
|---|---|---|---|---|---|---|
astro-ph/0403574
|
Yann Alibert
|
Migration and giant planet formation
|
astro-ph
|
We extend the core-accretion model of giant gaseous planets by Pollack et al.
(\cite{P96}) to include migration, disc evolution and gap formation. Starting
with a core of a fraction of an Earth's mass located at 8 AU, we end our
simulation with the onset of runaway gas accretion when the planet is at 5.5 AU
1 Myr later. This timescale is about a factor ten shorter than the one found by
Pollack et al. (\cite{P96}) even though the disc was less massive initially and
viscously evolving. Other initial conditions can lead to even shorter
timescales. The reason for this speed-up is found to result from the fact that
a moving planet does not deplete its feeding zone to the extend of a static
planet. Thus, the uncomfortably long formation timescale associated with the
core-accretion scenario can be considerably reduced and brought in much better
agreement with the typical disc lifetimes inferred from observations of young
circumstellar discs.
|
No Label
|
No Label
|
astro-ph/0302381
|
Soker Noam
|
Problems in suppressing cooling flows in clusters of galaxies by global
heat conduction
|
astro-ph
|
I use a simple analytical model to show that simple heat conduction models
cannot significantly suppress cluster cooling flows. I build a static medium
where heat conduction globally balances radiative cooling, and then perturb it.
I show that a perturbation extending over a large fraction of the cooling flow
region will grow to the non-linear regime within a Hubble time. Such
perturbations are reasonable in clusters which frequently experience mergers
and/or AGN activity. This result strengthens previous findings which show that
a steady solution does not exist for a constant heat conduction coefficient.
|
No Label
|
No Label
|
astro-ph/0402016
|
Jens Kauffmann
|
Disappearance of N2H+ from the Gas Phase in the Class 0 Protostar IRAM
04191
|
astro-ph
|
We present a high-resolution spectroscopic study of the envelope of the young
Class 0 protostar IRAM 04191+1522 in Taurus. N2H+(1-0) observations with the
Plateau de Bure Interferometer and the IRAM 30m telescope demonstrate that the
molecular ion N2H+ disappears from the gas phase in the inner envelope (r <
1600 AU, n(H2) > 4-7 * 1e5 cm-3). This may result from N2 depletion on polar
ice mantles and enhanced grain chemistry.
|
No Label
|
No Label
|
0812.1220
|
Andrew West
|
A First Look at Rotation in Inactive Late-Type M Dwarfs
|
astro-ph
|
We have examined the relationship between rotation and activity in 14
late-type (M6-M7) M dwarfs, using high resolution spectra taken at the W.M.
Keck Observatory and flux-calibrated spectra from the Sloan Digital Sky Survey.
Most were selected to be inactive at a spectral type where strong H-alpha
emission is quite common. We used the cross-correlation technique to quantify
the rotational broadening; six of the stars in our sample have vsini > 3.5
km/s. Our most significant and perplexing result is that three of these stars
do not exhibit H-alpha emission, despite rotating at velocities where previous
work has observed strong levels of magnetic field and stellar activity. Our
results suggest that rotation and activity in late-type M dwarfs may not always
be linked, and open several additional possibilities including a
rotationally-dependent activity threshold, or a possible dependence on stellar
parameters of the Rossby number at which magnetic/activity "saturation" takes
place in fully convective stars.
|
No Label
|
No Label
|
0808.1056
|
Michael Famiano
|
Effects of Beta-Decays of Excited-State Nuclei on the Astrophysical
r-Process
|
astro-ph
|
A rudimentary calculation is employed to evaluate the possible effects of
beta- decays of excited-state nuclei on the astrophysical r-process.
Single-particle levels calculated with the FRDM are adapted to the calculation
of beta-decay rates of these excited-state nuclei. Quantum numbers are
determined based on proximity to Nilson model levels. The resulting rates are
used in an r-process network calculation in which a supernova hot-bubble model
is coupled to an extensive network calculation including all nuclei between the
valley of stability and the neutron drip line and with masses A<284. Beta-decay
rates are included as functional forms of the environmental temperature. While
the decay rate model used is simple and phenomenological, it is consistent
across all 3700 nuclei involved in the r-process network calculation. This
represents an approximate first estimate to gauge the possible effects of
excited-state beta-decays on r-process freeze-out abundances.
|
No Label
|
No Label
|
astro-ph/0512484
|
Andrew R. Liddle
|
Model selection as a science driver for dark energy surveys
|
astro-ph
|
A key science goal of upcoming dark energy surveys is to seek time evolution
of the dark energy. This problem is one of {\em model selection}, where the aim
is to differentiate between cosmological models with different numbers of
parameters. However, the power of these surveys is traditionally assessed by
estimating their ability to constrain parameters, which is a different
statistical problem. In this paper we use Bayesian model selection techniques,
specifically forecasting of the Bayes factors, to compare the abilities of
different proposed surveys in discovering dark energy evolution. We consider
six experiments -- supernova luminosity measurements by the Supernova Legacy
Survey, SNAP, JEDI, and ALPACA, and baryon acoustic oscillation measurements by
WFMOS and JEDI -- and use Bayes factor plots to compare their statistical
constraining power. The concept of Bayes factor forecasting has much broader
applicability than dark energy surveys.
|
No Label
|
No Label
|
0704.2195
|
C. W. Engelbracht
|
Absolute Calibration and Characterization of the Multiband Imaging
Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 micron
Calibration
|
astro-ph
|
We present the stellar calibrator sample and the conversion from instrumental
to physical units for the 24 micron channel of the Multiband Imaging Photometer
for Spitzer (MIPS). The primary calibrators are A stars, and the calibration
factor based on those stars is 4.54*10^{-2} MJy sr^{-1} (DN/s)^{-1}, with a
nominal uncertainty of 2%. We discuss the data-reduction procedures required to
attain this accuracy; without these procdures, the calibration factor obtained
using the automated pipeline at the Spitzer Science Center is 1.6% +/- 0.6%
lower. We extend this work to predict 24 micron flux densities for a sample of
238 stars which covers a larger range of flux densities and spectral types. We
present a total of 348 measurements of 141 stars at 24 micron. This sample
covers a factor of ~460 in 24 micron flux density, from 8.6 mJy up to 4.0 Jy.
We show that the calibration is linear over that range with respect to target
flux and background level. The calibration is based on observations made using
3-second exposures; a preliminary analysis shows that the calibration factor
may be 1% and 2% lower for 10- and 30-second exposures, respectively. We also
demonstrate that the calibration is very stable: over the course of the
mission, repeated measurements of our routine calibrator, HD 159330, show a
root-mean-square scatter of only 0.4%. Finally, we show that the point spread
function (PSF) is well measured and allows us to calibrate extended sources
accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a
sample of nearby galaxies are identical within the uncertainties.
|
No Label
|
No Label
|
astro-ph/0703571
|
Eduardo Rozo
|
Cosmological Constraints from SDSS maxBCG Cluster Abundances
|
astro-ph
|
We perform a maximum likelihood analysis of the cluster abundance measured in
the SDSS using the maxBCG cluster finding algorithm. Our analysis is aimed at
constraining the power spectrum normalization $\sigma_8$, and assumes flat
cosmologies with a scale invariant spectrum, massless neutrinos, and CMB and
supernova priors Omega_m*h^2=0.128+/-0.01 and h=0.72+/-0.05 respectively.
Following the method described in the companion paper Rozo et al. 2007, we
derive \sigma_8=0.92+/-0.10$ (1-sigma) after marginalizing over all major
systematic uncertainties. We place strong lower limits on the normalization,
sigma_8>0.76 (95% CL) (>0.68 at 99% CL). We also find that our analysis favors
relatively low values for the slope of the Halo Occupation Distribution (HOD),
alpha=0.83+/-0.06. The uncertainties of these determinations will substantially
improve upon completion of an ongoing campaign to estimate dynamical, weak
lensing, and X-ray cluster masses in the SDSS maxBCG cluster sample.
|
No Label
|
No Label
|
astro-ph/0508064
|
Roberto Maiolino
|
First detection of [CII]158um at high redshift: vigorous star formation
in the early universe
|
astro-ph
|
We report the detection of the 2P_3/2 -> 2P_1/2 fine-structure line of C+ at
157.74 micron in SDSSJ114816.64+525150.3 (hereafter J1148+5251), the most
distant known quasar, at z=6.42, using the IRAM 30-meter telescope. This is the
first detection of the [CII] line at high redshift, and also the first
detection in a Hyperluminous Infrared Galaxy (L_FIR > 10^13 Lsun). The [CII]
line is detected at a significance level of 8 sigma and has a luminosity of 4.4
x 10^9 Lsun. The L_[CII]/L_FIR ratio is 2 x 10^-4, about an order of magnitude
smaller than observed in local normal galaxies and similar to the ratio
observed in local Ultraluminous Infrared Galaxies. The [CII] line luminosity
indicates that the host galaxy of this quasar is undergoing an intense burst of
star formation with an estimated rate of ~3000 Msun/yr. The detection of C+ in
SDSS J1148+5251 suggests a significant enrichment of metals at z ~ 6 (age of
the universe ~870 Myr), although the data are consistent with a reduced carbon
to oxygen ratio as expected from chemical evolutionary models of the early
phases of galaxy formation.
|
No Label
|
No Label
|
astro-ph/0004250
|
H. M. Bryce
|
Microlensing extended stellar sources
|
astro-ph
|
We have developed a code to compute multi-colour microlensing lightcurves for
extended sources, including the effects of limb darkening and photospheric star
spots as a function of spot temperature, position, size and lens trajectory.
Our model also includes the effect of structure within the spot and rotation of
the stellar source.
Our results indicate that star spots generally give a clear signature only
for transit events. Moreover, this signature is strongly suppressed by limb
darkening for spots close to the limb -- although such spots can be detected by
favourable lens trajectories.
|
No Label
|
No Label
|
astro-ph/0309707
|
Steven Balbus
|
Ambipolar Diffusion in the Magnetorotational Instability
|
astro-ph
|
The effects of ambipolar diffusion on the linear stability of weakly ionised
accretion discs are examined. Earlier work on this topic has focused on axial
magnetic fields and perturbation wavenumbers. We consider here more general
field and wavenumber geometries, and find that qualitatively new results are
obtained. Provided a radial wavenumber and azimuthal field are present along
with their axial counterparts, ambipolar diffusion will always be
destabilising, with unstable local modes appearing at well-defined wavenumber
bands. The wavenumber corresponding to the maximum growth rate need not, in
general, lie along the vertical axis. Growth rates become small relative to the
local angular velocity when the ion-neutral collision time exceeds the orbital
time. In common with Hall electromotive forces, ambipolar diffusion
destabilises both positive and negative angular velocity gradients. In at least
some cases, therefore, uniformly rotating molecular cloud cores may reflect the
marginally stable state of the ambipolar magnetorotational instability.
|
No Label
|
No Label
|
astro-ph/9711030
|
Thomas Buchert
|
A fresh look at the adhesion approximation
|
astro-ph
|
I report on a systematic derivation of the phenomenological ``adhesion
approximation'' from gravitational instability together with a brief evaluation
of the related status of analytical modeling of large-scale structure.
|
No Label
|
No Label
|
astro-ph/0602596
|
Naveen Reddy
|
Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from
Spitzer MIPS Observations
|
astro-ph
|
Using very deep Spitzer/MIPS 24 micron observations, we present an analysis
of the bolometric luminosities (L[bol]) and UV extinction properties of more
than 200 spectroscopically identified, optically selected (UGR) z~2 galaxies in
the GOODS-N field. The large spectroscopic sample is supplemented with near-IR
selected (BzK/DRG) galaxies and submm sources at similar redshifts in the same
field, providing a representative collection of relatively massive (M*>1e10
Msun) galaxies at high redshifts. We focus on the redshift range 1.5-2.6, where
MIPS is sensitive to the strength of the mid-IR PAH features in the galaxy
spectra (rest-frame 5-8.5 micron). We demonstrate, using stacked X-ray data and
a subset of galaxies with H-alpha measurements, that L(5-8.5) provides a
reliable estimate of L(IR) for most star forming galaxies at z~2. The range of
L(IR) in the samples considered extends from ~1e10 to >1e12 Lsun, with a mean
of 2e11 Lsun. Using 24 micron observations to infer dust extinction in high
redshift galaxies, we find that, as in the local universe, the obscuration
(L[IR]/L[1600]) is strongly dependent on L(bol), and ranges in value from <1 to
\~1000. However, the obscuration is ~10 times smaller at a given L(bol) at z~2
than at z=0. We show that the values of L(IR) and obscuration inferred from the
UV spectral slope beta generally agree well with the values inferred from
L(5-8.5) for L(bol)<1e12 Lsun. For galaxies with L(bol)>1e12 Lsun, it is common
for UV-based estimates to underpredict L(IR) by a factor of ~10-100. Using the
specific SFR as a proxy for cold gas fraction, we find a wide range in the
evolutionary state of galaxies at z~2, from those which have just begun to form
stars to those which have already accumulated most of their stellar mass and
are about to become, or already are, passively-evolving. [Abridged]
|
No Label
|
No Label
|
astro-ph/0611921
|
Liang Gao
|
Assembly bias in the clustering of dark matter haloes
|
astro-ph
|
We use a very large simulation of structure growth in a LCDM universe -- the
Millennium Simulation -- to study assembly bias, the fact that the large-scale
clustering of haloes of given mass varies significantly with their assembly
history. We extend earlier work based on the same simulation by superposing
results for redshifts from 0 to 3, by defining a less noisy estimator of
clustering amplitude, and by considering halo concentration, substructure mass
fraction and spin, as well as formation time, as additional parameters. These
improvements lead to results with less noise than previous studies and covering
a wider range of halo masses and structural properties. We find significant and
significantly different assembly bias effects for all the halo properties we
consider, although in all cases the dependences on halo mass and on redshift
are adequately described as a dependence on equivalent peak height nu(M,z). The
nu-dependences for different halo properties differ qualitatively and are not
related as might naively be expected given the relations between formation
time, concentration, substructure fraction and spin found for the halo
population as a whole. These results suggest that it will be difficult to build
models for the galaxy populations of dark haloes which can robustly relate the
amplitude of large-scale galaxy clustering to that for mass clustering at
better than the 10% level.
|
No Label
|
No Label
|
astro-ph/0502476
|
Curtis S. Cooper
|
Dynamic Meteorology at the Photosphere of HD 209458b
|
astro-ph
|
We calculate the meteorology of the close-in transiting extrasolar planet HD
209458b using a global, three-dimensional atmospheric circulation model.
Dynamics are driven by perpetual irradiation of one hemisphere of this tidally
locked planet. The simulation predicts global temperature contrasts of ~500 K
at the photosphere and the development of a steady superrotating jet. The jet
extends from the equator to mid-latitudes and from the top model layer at 1
mbar down to 10 bars at the base of the heated region. Wind velocities near the
equator exceed 4 km/s at 300 mbar. The hottest regions of the atmosphere are
blown downstream from the substellar point by 60 degrees of longitude. We
predict from these results a factor of ~2 ratio between the maximum and minimum
observed radiation from the planet over a full orbital period, with peak
infrared emission preceding the time of the secondary eclipse by ~14 hours.
|
No Label
|
No Label
|
astro-ph/9609119
| null |
Causal Thermodynamics in Relativity
|
astro-ph
|
I review the causal relativistic thermodynamics developed by Israel and
Stewart, and discuss some applications in cosmology and astrophysics. The
lectures begin with an overview of relativistic fluid dynamics (in a covariant
formalism) and equilibrium thermodynamics. Causal bulk viscosity in cosmology
is considered in detail, including some new results.
|
No Label
|
No Label
|
0810.2111
|
Leonid V. Verozub
|
On the stability of compact supermassive objects
|
astro-ph
|
Proceeded from the gravitation equations proposed by one of authors it was
argued in a previous paper that there can exist supermassive compact
configurations of degenerated Fermi-gas without events horizon. In the present
paper we consider the stability of these objects by method like the one used in
the theory of stellar structure. It is shown that the configurations with an
adiabatic equation of state with the power 4/3 are stable.
|
No Label
|
No Label
|
astro-ph/9212004
|
Paul Steinhardt
|
Recent advances in extended inflationary cosmology
|
astro-ph
|
Extended inflation is a promising new approach to implementing the
inflationary universe scenario. This paper reviews recent advances including a
new, more robust mechanism for ending extended inflation, a new prediction for
the density fluctuation spectrum generated by extended inflation, and the
discovery that extended inflation can produce gravitational waves that can
significantly add to the cosmic background anisotropy. Invited Talk at the
Journees Relativistes, Amsterdam, May 14-16, 1992
|
No Label
|
No Label
|
astro-ph/9509037
|
Ewa Szuszkiewicz
|
The Observational Appearance of Slim Accretion Disks
|
astro-ph
|
We reexamine the hypothesis that the optical/UV/soft X-ray continuum of
Active Galactic Nuclei is thermal emission from an accretion disk. Previous
studies have shown that fitting the spectra with the standard, optically thick
and geometrically thin accretion disk models often led to luminosities which
contradict the basic assumptions adopted in the standard model. There is no
known reason why the accretion rates in AGN should not be larger than the thin
disk limit. In fact, more general, slim accretion disk models are
self-consistent even for moderately super-Eddington luminosities. We calculate
here spectra from a set of thin and slim, optically thick accretion disks. We
discuss the differences between the thin and slim disk models, stressing the
implications of these differences for the interpretation of the observed
properties of AGN. We found that the spectra can be fitted not only by models
with a high mass and a low accretion rate (as in the case of thin disk fitting)
but also by models with a low mass and a high accretion rate. In the first case
fitting the observed spectra in various redshift categories gives black hole
masses around 10^9 solar masses for a wide range of redshifts, and for
accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the
accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges
from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate,
the spectra of the high-accretion-rate disks extend into the soft X-rays. A
comparison with observations shows that such disks could produce the soft X-ray
excesses claimed in some AGNs. We show also that the sequence of our models
with fixed mass and different accretion rates can explain the time evolution of
the observed spectra in Fairall 9.
|
No Label
|
No Label
|
astro-ph/0005110
|
Alexander Heger
|
Presupernova Evolution of Rotating Massive Stars II: -- Evolution of the
Surface Properties
|
astro-ph
|
We investigate the evolution of the surface properties of models for rotating
massive stars, i.e., their luminosities, effective temperatures, surface
rotational velocities, and surface abundances of all isotopes, from the zero
age main sequence to the supernova stage. Our results are based on the grid of
stellar models by Heger, Langer, & Woosley, which covers solar metallicity
stars in the initial mass range 8 - 25 solar masses. Results are parameterized
by initial mass, initial rotational velocity and major uncertainties in the
treatment of the rotational mixing inside massive stars.
Rotationally induced mixing processes widen the main sequence and increase
the core hydrogen burning lifetime, similar to the effects of convective
overshooting. It can also significantly increase the luminosity during and
after core hydrogen burning, and strongly affects the evolution of the
effective temperature. Our models predict surface rotational velocities for
various evolutionary stages, in particular for blue supergiants, red
supergiants, and for the immediate presupernova stage.
We discuss the changes of the surface abundances due to rotationally induced
mixing for main sequence and post main sequence stars. We single out two
characteristics by which the effect of rotational mixing can be distinguished
from that of massive close binary mass transfer, the only alternative process
leading to non-standard chemical surface abundances in massive stars. A
comparison with observed abundance anomalies in various types of massive stars
supports the concept of rotational mixing in massive stars and indicates that
it is responsible for most of the observed abundance anomalies.
|
No Label
|
No Label
|
0811.4751
|
Tommi Koskinen
|
The Upper Atmosphere of HD17156b
|
astro-ph
|
HD17156b is a newly-found transiting extrasolar giant planet (EGP) that
orbits its G-type host star in a highly eccentric orbit (e~0.67) with an
orbital semi-major axis of 0.16 AU. Its period, 21.2 Earth days, is the longest
among the known transiting planets. The atmosphere of the planet undergoes a
27-fold variation in stellar irradiation during each orbit, making it an
interesting subject for atmospheric modelling. We have used a three-dimensional
model of the upper atmosphere and ionosphere for extrasolar gas giants in order
to simulate the progress of HD17156b along its eccentric orbit. Here we present
the results of these simulations and discuss the stability, circulation, and
composition in its upper atmosphere. Contrary to the well-known transiting
planet HD209458b, we find that the atmosphere of HD17156b is unlikely to escape
hydrodynamically at any point along the orbit, even if the upper atmosphere is
almost entirely composed of atomic hydrogen and H+, and infrared cooling by H3+
ions is negligible. The nature of the upper atmosphere is sensitive to to the
composition of the thermosphere, and in particular to the mixing ratio of H2,
as the availability of H2 regulates radiative cooling. In light of different
simulations we make specific predictions about the thermosphere-ionosphere
system of HD17156b that can potentially be verified by observations.
|
No Label
|
No Label
|
astro-ph/9907447
|
Dallas Kennedy
|
Simple Seismic Tests of the Solar Core
|
astro-ph
|
A model-independent reconstruction of mechanical profiles (density, pressure)
of the solar interior is outlined using the adiabatic sound speed and bouyancy
frequency profiles. These can be inferred from helioseismology if both p- and
g-mode frequencies are measured. A simulated reconstruction is presented using
a solar model bouyancy frequency and available sound speed data.
|
No Label
|
No Label
|
astro-ph/0309299
|
Cristian Vignali
|
Restless Quasar Activity: From BeppoSAX to Chandra and XMM-Newton
|
astro-ph
|
We briefly review some of the progress made in the last decade in the study
of the X-ray properties of the quasar population from the luminous, local
objects observed by BeppoSAX to the large, rapidly increasing population of z>4
quasars detected by Chandra and XMM-Newton in recent years.
|
No Label
|
No Label
|
0712.2759
|
Eli Dwek
|
Infrared and X-Ray Evidence for Circumstellar Grain Destruction by the
Blast Wave of Supernova 1987A
|
astro-ph
|
Multiwavelength observations of supernova remnant (SNR) 1987A show that its
morphology and luminosity are rapidly changing at X-ray, optical, infrared, and
radio wavelengths as the blast wave from the explosion expands into the
circumstellar equatorial ring, produced by mass loss from the progenitor star.
The observed infrared (IR) radiation arises from the interaction of dust grains
that formed in mass outflow with the soft X-ray emitting plasma component of
the shocked gas. Spitzer IRS spectra at 5 - 30 microns taken on day 6190 since
the explosion show that the emission arises from ~ 1.1E-6 Msun of silicate
grains radiating at a temperature of ~180+20-15 K. Subsequent observations on
day 7137 show that the IR flux had increased by a factor of 2 while maintaining
an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX)
is consistent with that of a dusty plasma with standard Large Magellanic Cloud
dust abundances. IRX has decreased by a factor of ~ 2 between days 6190 and
7137, providing the first direct observation of the ongoing destruction of dust
in an expanding SN blast wave on dynamic time scales. Detailed models
consistent with the observed dust temperature, the ionization timescale of the
soft X-ray emission component, and the evolution of IRX suggest that the
radiating silicate grains are immersed in a 3.5E6 K plasma with a density of
(0.3-1)E4 cm^{-3}, and have a size distribution that is confined to a narrow
range of radii between 0.023 and 0.22 microns. Smaller grains may have been
evaporated by the initial UV flash from the supernova.
|
No Label
|
No Label
|
astro-ph/9601148
|
Peter Hauschildt
|
Non-LTE Treatment of Fe II in Astrophysical Plasmas
|
astro-ph
|
We describe our implementation of an extremely detailed model atom of singly
ionized iron for NLTE computations in static and moving astrophysical plasmas.
Our model atom includes 617 levels, 13675 primary permitted transitions and up
to 1.2 million secondary transitions. Our approach guarantees that the total
iron opacity is included at the correct wavelength with reasonable memory and
CPU requirements. We find that the lines saturate the wavelength space, such
that special wavelength points inserted along the detailed profile functions
may be replaced with a statistical sampling method. We describe the results of
various test calculations for novae and supernovae.
|
No Label
|
No Label
|
astro-ph/9409002
|
Wyn Evans
|
Microlensing Maps for the Galactic Bulge
|
astro-ph
|
Microlensing maps -- that is, contours of equal numbers of events per $10^6$
source stars -- are provided for the inner Galaxy under two alternative
hypotheses : (1) the bulge is an oblate axisymmetric spheroid or (2) the bulge
is a prolate bar. Oblate spheroids yield a total of $\sim 12$ events per year
per $10^6$ stars at Baade's Window ($\sim 15$ events if the disk is maximal).
The event rate is slightly lower for prolate bars viewed at $\sim 45^\circ$ and
the maps have a characteristic asymmetry between positive and negative
longitudes. Prolate bars can yield mild amplifications of the event rate if
viewed almost down the long axis. The disk provides the dominant lensing
population on the bulge major axis for $|\ell | \gta 6^\circ$. Measurements of
the rate at major axis windows can test for disk dark matter or maximal disk
models.
|
No Label
|
No Label
|
0812.0505
|
Verena Baumgartner
|
Modeling the Local Warm/Hot Bubble
|
astro-ph
|
In this paper we review the modeling of the Local Bubble (LB) with special
emphasis on the progress we have made since the last major conference "The
Local Bubble and Beyond (I)" held in Garching in 1997. Since then new insight
was gained into the possible origin of the LB, with a moving group crossing its
volume during the last 10 - 15 Myr being most likely responsible for creating a
local cavity filled with hot recombining gas. Numerical high resolution 3D
simulations of a supernova driven inhomogeneous interstellar medium show that
we can reproduce both the extension of the LB and the OVI column density in
absorption measured with FUSE for a LB age of 13.5 - 14.5 Myr. We further
demonstrate that the LB evolves like an ordinary superbubble expanding into a
density stratified medium by comparing analytical 2D Kompaneets solutions to
NaI contours, representing the extension of the local cavity. These results
suggest that LB blow-out into the Milky Way halo has occurred roughly 5 Myr
ago.
|
No Label
|
No Label
|
0707.1533
|
Hugo Martel
|
The Fate of Dwarf Galaxies in Clusters and the Origin of Intracluster
Stars. I. Isolated Clusters
|
astro-ph
|
The main goal of this paper is to compare the relative importance of
destruction by tides, vs. destruction by mergers, in order to assess if tidal
destruction of dwarf galaxies in clusters is a viable scenario for explaining
the origin of intracluster stars. We have designed a simple algorithm for
simulating the evolution of isolated clusters. The distribution of galaxies in
the cluster is evolved using a direct gravitational N-body algorithm combined
with a subgrid treatment of physical processes such as mergers, tidal
disruption, and galaxy harassment. Using this algorithm, we have performed a
total of 227 simulations. Our main results are (1) destruction of dwarf
galaxies by mergers dominates over destruction by tides, and (2) the
destruction of dwarf galaxies by tides is sufficient to explain the observed
intracluster light in clusters.
|
No Label
|
No Label
|
astro-ph/0509886
|
Brent Buckalew
|
Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet,
Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations
|
astro-ph
|
We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and
radio images of 14 radio-selected objects in M 33. These objects are thought to
represent the youngest phase of star cluster formation. We have detected the
majority of cluster candidates in M 33 at all wavelengths. From the near-IR
images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and
stellar masses of 10^3-10^4 M_solar. We have generated spectral energy
distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these
SEDs, we have modeled the dust emission around these star clusters to determine
the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters'
local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV
images are similar to within a factor of 2 or 3. These results suggest that
eleven of the fourteen radio-selected objects are optically-visible young star
clusters with a surrounding H II region, that two are background objects,
possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.
|
No Label
|
No Label
|
0711.4790
|
Diego Altamirano
|
Millihertz Oscillation Frequency Drift Predicts the Occurrence of Type I
X-ray Bursts
|
astro-ph
|
Millihertz quasi-periodic oscillations reported in three neutron-star low
mass X-ray binaries have been suggested to be a mode of marginally stable
nuclear burning on the neutron star surface. In this Letter, we show that close
to the transition between the island and the banana state, 4U~1636--53 shows
mHz QPOs whose frequency systematically decreases with time until the
oscillations disappear and a Type I X-ray burst occurs. There is a strong
correlation between the QPO frequency $\nu$ and the occurrence of X-ray bursts:
when $\nu\gtrsim9$ mHz no bursts occur, while $\nu\lesssim9$ mHz does allow the
occurrence of bursts. The mHz QPO frequency constitutes the first identified
observable that can be used to predict the occurrence of X-ray bursts. If a
systematic frequency drift occurs, then a burst happens within a few
kilo-seconds after $\nu$ drops below 9 mHz. This observational result confirms
that the mHz QPO phenomenon is intimately related with the processes that lead
to a thermonuclear burst.
|
No Label
|
No Label
|
astro-ph/0402072
|
Nectaria Gizani
|
A multiband study of Hercules A. I. ROSAT observations of the
intracluster medium
|
astro-ph
|
We have made ROSAT PSPC and HRI X-ray observations to study the intracluster
gas surrounding the powerful radio source Hercules A. The cluster is luminous
in X-rays (L_bol = 4.8x10^37 W), and the host of the radio source is the
central dominating galaxy of the cluster. The X-ray surface brightness profile
is well fitted by a modified King beta model, but the cluster is elongated
parallel to the radio source, especially on the scale of the radio lobes. Fits
to individual quadrants give a core radius 50 per cent larger along the radio
axis. Part of this elongation appears to be associated with enhanced X-ray
emission superimposed on the outer radio lobes. There are no obvious
depressions in the X-ray emission coincident with the radio lobes, as expected
if the relativistic plasma displaces the ICM. However, we show that these
depressions may be quite weak. From the surface brightness profile for the PSPC
data the X-ray emission extends out to ~ 2.2 Mpc radius. In the absence of the
powerful jets, we would expect a cooling flow at the centre of the cluster, but
currently it must be substantially disturbed by the expansion of the radio
lobes. The PSPC spectrum reveals a cool component of the ICM with 0.5 < kT < 1
keV. The central cooling time could be as low as 2 Gyr if the cool component is
centrally concentrated, otherwise it is around 6 Gyr. The modelled central
electron density is typical for modest cooling flows. Finally, we have detected
faint X-ray emission from a compact central source.
|
No Label
|
No Label
|
0811.4750
|
Dami\'an Mast
|
Is J 133658.3-295105 a Radio Source at z >= 1.0 or at the Distance of M
83?
|
astro-ph
|
We present Gemini optical imaging and spectroscopy of the radio source J
133658.3-295105. This source has been suggested to be the core of an FR II
radio source with two detected lobes. J 133658.3-295105 and its lobes are
aligned with the optical nucleus of M 83 and with three other radio sources at
the M 83 bulge outer region. These radio sources are neither supernova remnants
nor H II regions. This curious configuration prompted us to try to determine
the distance to J 133658.3-295105. We detected H_alpha emission redshifted by ~
130 km s^-1 with respect to an M 83 H II region 2.5" east-southeast of the
radio source. We do not detect other redshifted emission lines of an optical
counterpart down to m_i = 22.2 +/- 0.8. Two different scenarios are proposed:
the radio source is at z >= 2.5, a much larger distance than the previously
proposed lower limit z >= 1.0, or the object was ejected by a gravitational
recoil event from the M 83 nucleus. This nucleus is undergoing a strong
dynamical evolution, judging from previous three-dimensional spectroscopy.
|
No Label
|
No Label
|
astro-ph/0309604
|
Wang Wei
|
Does the Iron K$_{\alpha}$ Line of Active Galactic Nuclei Arise from the
Cerenkov Line-like Radiation?
|
astro-ph
|
When thermal relativistic electrons with isotropic distribution of velocities
move in a gas region, or impinge upon the surface of a cloud that consists of a
dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic
emission lines -- the Cerenkov line-like radiation. This newly recognized
emission mechanism may find wide applications in high-energy astrophysics. In
this paper, we tentatively adopt this new line emission mechanism to discuss
the origin of iron K$_{\alpha}$ feature of AGNs. Motivation of this research is
to attempt a solution to a problem encountered by the ``disk-fluorescence
line'' model, i.e. the lack of temporal response of the observed iron
K$_{\alpha}$ line flux to the changes of the X-ray continuum flux. If the
Cerenkov line emission is indeed responsible significantly for the iron
K$_{\alpha}$ feature, the conventional scenario around the central supermassive
black holes of AGNs would need to be modified to accommodate more energetic,
more violent and much denser environments than previously thought.
|
No Label
|
No Label
|
astro-ph/9908261
|
Christian Kaiser
|
Multi-frequency observations as a key to source and environment
parameters of FRII objects
|
astro-ph
|
Our knowledge of the environments of radio-loud AGN is still sketchy.
However, to understand the jet phenomenon it is important to know about the
properties of the surroundings in which jets are formed and evolve. Here I
present an analytical model of the radio surface brightness distribution of the
large scale structure of FRII-type radio sources. The `virtual maps' resulting
from this model can be compared with observed maps to obtain estimates for a
range of source properties from the model. These properties include parameters
describing the gas density distribution of the source environment, the energy
transport rate of the jets and the orientation angle of the source jet axis
with respect to the line of sight. The model is tested using radio maps of
Cygnus A for which there are independent measurements of some of these
parameters available in the literature. The model estimates agree well with
these observations. Varying the resolution of the radio maps used in
thecomparison does not change the results significantly.
|
No Label
|
No Label
|
astro-ph/0109090
|
Marc R. Gagne
|
Mass Loss and Magnetospheres: X-rays from Hot Stars and Young Stellar
Objects
|
astro-ph
|
High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars
obtained during the first year of the Chandra mission are providing important
clues about the mechanisms which produce X-rays on very young stars. For zeta
Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles,
line ratios, and derived temperature distribution suggest that the X-rays are
produced throughout the wind via instability-driven wind shocks. For some less
luminous OB stars, like theta^1 Ori C (O7 V) and tau Sco (B0 V), the line
profiles are symmetric and narrower. The presence of time-variable emission and
very high-temperature lines in theta^1 Ori C and tau Sco suggest that
magnetically confined wind shocks may be at work. The grating spectrum of the
classical T-Tauri star TW Hya is remarkable because the forbidden-line emission
of He-like Ne IX and O VII is very weak, implying that the X-ray emitting
region is very dense, n = 6E+12 cgs, or that the X-rays are produced very close
to the ultraviolet hotspot at the base of an accretion funnel. ACIS light
curves and spectra of flares and low-mass and high-mass young stellar objects
in Orion and rho Ophiuchus further suggest that extreme magnetic activity is a
general property of many very young stars.
|
No Label
|
No Label
|
astro-ph/0105003
|
Carl R. Gwinn
|
Small-Scale Variations of HI Spectra from Interstellar Scintillatio
|
astro-ph
|
I suggest that radio-wave scattering by the interstellar plasma, in
combination with subsonic gradients in the Doppler velocity of interstellar HI,
is responsible for the observed small-scale variation in HI absorption spectra
of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can
produce the observed variations. I suggest observational tests to distinguish
between this model and the traditional picture of small-scale opacity
variations from cloudlets.
|
No Label
|
No Label
|
astro-ph/0008031
|
Bram Boroson
|
Discovery of mHz UV Quasiperiodic Oscillations in Hercules X-1
|
astro-ph
|
Observations of the ultraviolet continuum of the X-ray binary system Her
X-1/HZ Herculis with the Space Telescope Imaging Spectrograph on the Hubble
Space Telescope show quasiperiodic oscillations (QPOs) at frequencies of 8+/-2
and 43+/-2 mHz, with rms amplitudes of 2 and 4% of the steady flux.
Observations with the Keck telescope confirm the presence of the higher
frequency QPO in the optical continuum, with a rms amplitude of 1.6+/-0.2%. The
QPOs are most prominent in the HST data near phi=0.5 (where phi=0 is the middle
of the X-ray eclipse), suggesting that they arise not in the accretion disk but
on the X-ray heated face of the companion star. We discuss scenarios in which
the companion star reprocesses oscillations in the disk which are caused by
either Keplerian rotation or a beat frequency between the neutron star spin and
Keplerian rotation at some radius in the accretion disk.
|
No Label
|
No Label
|
astro-ph/0501468
|
Banibrata Mukhopadhyay
|
Hydrodynamic Turbulence in Accretion Disks
|
astro-ph
|
Turbulent viscosity in cold accretion disks is likely to be hydrodynamic in
origin. We investigate the growth of hydrodynamic perturbations in a small
region of a disk, which we model as a linear shear flow with Coriolis force,
between two parallel walls. Although there are no exponentially growing
eigenmodes in this system, because of the non-normal nature of the modes, it is
possible to have a large transient growth in the energy of certain
perturbations. For a constant angular momentum disk, the energy grows by more
than a factor of 1000 for a Reynolds number of only 1000, and so turbulence is
easily excited. For a Keplerian disk, the growth is more modest, and energy
growth by a factor of 1000 requires a Reynolds number of nearly a million.
Accretion disks have even larger Reynolds numbers than this. Therefore,
transient growth of perturbations could seed turbulence in such disks.
|
No Label
|
No Label
|
astro-ph/9712297
|
Filippo Frontera
|
Broad band X-ray spectral properties of Gamma-ray bursts with BeppoSAX
|
astro-ph
|
In about one year, five gamma-ray bursts were simultaneously observed with
the Wide Field Cameras and Gamma Ray Burst Monitor aboard the BeppoSAX
satellite. From some of them X-ray afterglow emission has been clearly detected
with the same satellite. In order to understand how GRB emission is related to
the X-ray afterglow, we are performing a systematic study of the spectral
properties of these events. We report here preliminary results of this study.
|
No Label
|
No Label
|
astro-ph/0112272
|
Maria Luisa Sarsa Sarsa
|
The ROSEBUD experiment at Canfranc: 2001 report
|
astro-ph
|
The ROSEBUD experiment for Direct Dark Matter detection settled in 1999 in
the Canfranc Underground Laboratory. The first phase of the experiment was
dedicated to the understanding and reduction of the radioactive background
following successive removals of the radioimpure materials. Sapphire (25g, 50g)
and germanium (67g) absorbers were used. Thresholds respectively lower than
1keV and 450 eV were achieved on these detectors. The second phase of the
experiment plans to use scintillating bolometers to discriminate between
recoiling nuclei and electrons. Prototypes using commercial CaWO4 (54g) and BGO
(46g) were designed for this purpose. While internal contamination was found
and identified in both targets, neutron calibrations revealed their high
discrimination power. A 6 keV threshold on the heat channel of the BGO
bolometer points out the interest of such a novel material, for Dark Matter
research on neutralinos having spin-dependent or spin-independent interactions.
|
No Label
|
No Label
|
0712.2297
|
Andrzej Niedzielski
|
A HET search for planets around evolved stars
|
astro-ph
|
We present our ongoing survey of ~1000 GK-giants with the 9.2-m Hobby-Eberly
Telescope in search for planets around evolved stars. The stars selected for
this survey are brighter than 11 mag and are located in the section of the
HR-diagram, which is approximately delimited by the main sequence, the
instability strip, and the coronal dividing line. We use the High Resolution
Spectrograph to obtain stellar spectra for radial velocity measurements with a
4-6 m/s precision. So far, the survey has discovered a planetary-mass companion
to the K0-giant HD 17092, and it has produced a number of plausible planet
candidates around other stars. Together with other similar efforts, our program
provides information on planet formation around intermediate mass main
sequence-progenitors and it will create the experimental basis with which to
study dynamics of planetary systems around evolving stars.
|
No Label
|
No Label
|
astro-ph/0611310
|
Paul A. Wiegert
|
Evidence for a colour dependence in the size distribution of main belt
asteroids
|
astro-ph
|
We present the results of a project to detect small (~1 km) main-belt
asteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observed
in 2 filters (MegaPrime g' and r') in order to compare the results in each
band. Owing to the observational cadence we did not observe the same asteroids
through each filter and thus do not have true colour information. However
strong differences in the size distributions as seen in the two filters point
to a colour-dependence at these sizes, perhaps to be expected in this regime
where asteroid cohesiveness begins to be dominated by physical strength and
composition rather than by gravity. The best fit slopes of the cumulative size
distributions (CSDs) in both filters tend towards lower values for smaller
asteroids, consistent with the results of previous studies. In addition to this
trend, the size distributions seen in the two filters are distinctly different,
with steeper slopes in r' than in g'. Breaking our sample up according to
semimajor axis, the difference between the filters in the inner belt is found
to be somewhat less pronounced than in the middle and outer belt, but the CSD
of those asteroids seen in the r' filter is consistently and significantly
steeper than in g' throughout. The CSD slopes also show variations with
semimajor axis within a given filter, particularly in r'. We conclude that the
size distribution of main belt asteroids is likely to be colour dependent at
kilometer sizes and that this dependence may vary across the belt.
|
No Label
|
No Label
|
astro-ph/0006423
|
Norbert Straumann
|
Reflections on Gravity
|
astro-ph
|
A pedagogical description of a simple ungeometrical approach to General
Relativity is given, which follows the pattern of well understood field
theories, such as electrodynamics. This leads quickly to most of the important
weak field predictions, as well as to the radiation damping of binary pulsars.
Moreover, certain consistency arguments imply that the theory has to be
generally invariant, and therefore one is bound to end up with Einstein's field
equations. Although this field theoretic approach, which has been advocated
repeatedly by a number of authors, starts with a spin-2 theory on Minkowski
spacetime, it turns out in the end that the flat metric is actually
unobservable, and that the physical metric is curved and dynamical.
Short sections are devoted to tensor-scalar generalizations, the mystery of
the vacuum energy density, and quintessence.
|
No Label
|
No Label
|
0710.4893
|
Robin Barnard
|
A Multi-coloured survey of NGC 253 with XMM-Newton
|
astro-ph
|
There is a large body of work that has used the excellent Chandra
observations of nearby galaxies with neglible low mass X-ray binary (LMXB)
populations. This has culminated in a ``Universal'' X-ray luminosity function
(XLF) for high mass X-ray binaries (HMXBs). However, a number of methods have
been used to convert from source intensities to luminosities when creating
these XLFs. We have taken advantage of the XMM-Newton observations of the
nearby starbursting spiral galaxy NGC 253 to test some of these methods. We
find the luminosities derived from these various methods to vary by a factor of
$\sim$3. We also find the most influential factor in the conversion from
intensity to luminosity to be the absorption. We therefore conclude that a more
consistent approach is required for determining the true Universal XLF for
HMXBs. Ideally, this would involve individual spectral fitting of each X-ray
source. Certainly, the line-of-sight absorption should be determined from the
observations rather than assuming Galactic absorption. We find the best
approach for obtaining an XLF from low-count data to be the splitting of the
X-ray sources into two or more intensity intervals, and obtaining a conversion
from intensity to flux for each group from spectral modelling of the summed
spectrum of that group.
|
No Label
|
No Label
|
astro-ph/0701907
|
Gustavo Bruzual A.
|
Studying stellar populations at high spectral resolution
|
astro-ph
|
I describe very briefly the new libraries of empirical spectra of stars
covering wide ranges of values of the atmospheric parameters Teff, log g,
[Fe/H], as well as spectral type, that have become available in the recent
past, among them the HNGSL, MILES, UVES-POP, ELODIE, and the IndoUS libraries.
I show the results of using the IndoUS and the HNGSL libraries, as well as an
atlas of theoretical model atmospheres, to build population synthesis models.
These libraries are complementary in spectral resolution and wavelength
coverage, and will prove extremely useful to describe spectral features
expected in galaxy spectra from the NUV to the NIR. The fits to observed galaxy
spectra using simple and composite stellar population models are discussed.
|
No Label
|
No Label
|
astro-ph/0410542
|
Thierry Pradier
|
ANTARES Status & Milestones : News from Deep-Sea
|
astro-ph
|
The ANTARES project aims to build a deep-sea Cherenkov Telescope for High
Energy Neutrino Astronomy located in the Mediterranean Sea. The experiment,
currently in the construction phase, has recently achieved an important
milestone : the operation of a prototype line and of a line with monitoring
instruments. These deployments allowed a thorough understanding of
environmental parameters.
|
No Label
|
No Label
|
astro-ph/0409574
|
David Rapetti
|
Constraining Dark Energy with X-ray Galaxy Clusters, Supernovae and the
Cosmic Microwave Background
|
astro-ph
|
We present new constraints on the evolution of dark energy from an analysis
of Cosmic Microwave Background, supernova and X-ray galaxy cluster data. Our
analysis employs a minimum of priors and exploits the complementary nature of
these data sets. We examine a series of dark energy models with up to three
free parameters: the current dark energy equation of state w_0, the early time
equation of state w_et and the scale factor at transition, a_t. From a combined
analysis of all three data sets, assuming a constant equation of state and that
the Universe is flat, we measure w_0=-1.05+0.10-0.12. Including w_et as a free
parameter and allowing a_t to vary over the range 0.5<a_t<0.95 where the data
sets have discriminating power, we measure w_0=-1.27+0.33-0.39 and
w_et=-0.66+0.44-0.62. We find no significant evidence for evolution in the dark
energy equation of state parameter with redshift. Marginal hints of evolution
in the supernovae data become less significant when the cluster constraints are
also included in the analysis. The complementary nature of the data sets leads
to a tight constraint on the mean matter density, Omega_m and alleviates a
number of other parameter degeneracies, including that between the scalar
spectral index n_s, the physical baryon density Omega_bh^2 and the optical
depth tau. This complementary nature also allows us to examine models in which
we drop the prior on the curvature. For non-flat models with a constant
equation of state, we measure w_0=-1.09+0.12-0.15 and Omega_de=0.70+-0.03. Our
analysis includes spatial perturbations in the dark energy fluid, assuming a
sound speed c_s^2 =1. For our most general dark energy model, not including
such perturbations would lead to spurious constraints on w_et which would be
tighter by approximately a factor two with the current data. (abridged)
|
No Label
|
No Label
|
astro-ph/9401012
|
There's Always Something...
|
Cosmic Temperature Fluctuations from Two Years of COBE DMR Observations
|
astro-ph
|
The first two years of COBE DMR observations of the CMB anisotropy are
analyzed and compared with our previously published first year results. The
results are consistent, but the addition of the second year of data increases
the precision and accuracy of the detected CMB temperature fluctuations. The
two-year 53 GHz data are characterized by RMS temperature fluctuations of
DT=44+/-7 uK at 7 degrees and DT=30.5+/-2.7 uK at 10 degrees angular
resolution. The 53X90 GHz cross-correlation amplitude at zero lag is
C(0)^{1/2}=36+/-5 uK (68%CL) for the unsmoothed 7 degree DMR data. A likelihood
analysis of the cross correlation function, including the quadrupole
anisotropy, gives a most likely quadrupole-normalized amplitude
Q_{rms-PS}=12.4^{+5.2}_{-3.3} uK (68% CL) and a spectral index
n=1.59^{+0.49}_{-0.55} for a power law model of initial density fluctuations,
P(k)~k^n. With n fixed to 1.0 the most likely amplitude is 17.4 +/-1.5 uK (68%
CL). Excluding the quadrupole anisotropy we find Q_{rms-PS}= 16.0^{+7.5}_{-5.2}
uK (68% CL), n=1.21^{+0.60}_{-0.55}, and, with n fixed to 1.0 the most likely
amplitude is 18.2+/-1.6 uK (68% CL). Monte Carlo simulations indicate that
these derived estimates of n may be biased by ~+0.3 (with the observed low
value of the quadrupole included in the analysis) and {}~+0.1 (with the
quadrupole excluded). Thus the most likely bias-corrected estimate of n is
between 1.1 and 1.3. Our best estimate of the dipole from the two-year DMR data
is 3.363+/-0.024 mK towards Galactic coordinates (l,b)= (264.4+/-0.2 degrees,
+48.1+/-0.4 degrees), and our best estimate of the RMS quadrupole amplitude in
our sky is 6+/-3 uK.
|
No Label
|
No Label
|
astro-ph/0011565
|
Jos de Bruijne
|
A Hipparcos study of the Hyades open cluster: Improved colour-absolute
magnitude and Hertzsprung-Russell diagrams
|
astro-ph
|
Hipparcos parallaxes fix distances to individual stars in the Hyades cluster
with an accuracy of 6%. We use the Hipparcos (and Tycho-2) proper motions,
which have a larger relative precision than the trigonometric parallaxes, to
derive ~3 times more precise distance estimates, by assuming that all members
share the same space motion. The improved parallaxes as a set are statistically
consistent with the Hipparcos parallaxes. The new parallaxes confirm that the
Hipparcos measurements are correlated on small angular scales, consistent with
the limits specified in the Hipparcos Catalogue, though with significantly
smaller `amplitudes' than claimed by Narayanan & Gould.
The colour-absolute magnitude diagram of the cluster based on the new paral-
laxes shows a well-defined main sequence with two gaps/turn-offs. These
features provide the first direct observational support of Boehm-Vitense's
prediction that (the onset of) surface convection in stars affects their B-V
colours. We present and discuss the theoretical HRD for an objectively defined
set of 88 high-fidelity members of the cluster as well as the delta Scuti star
theta^2 Tau, the giants delta^1, theta^1, epsilon, and gamma Tau, and the white
dwarfs V471 Tau and HD 27483 (all of which are also members). The precision
with which the new parallaxes place individual Hyades members in the Hertz-
sprung-Russell diagram is limited by (systematic) uncertainties related to the
transformations from observed colours and absolute magnitudes to effective
temperatures and luminosities. The new parallaxes provide stringent constraints
on the calibration of such transformations when combined with theoretical
stellar evolutionary modelling, tailored to the chemical composition and age of
the Hyades, over the large stellar mass range probed by Hipparcos.
|
No Label
|
No Label
|
astro-ph/9810147
|
Rosanne Di Stefano
|
A New Channel for the Detection of Planetary Systems Through
Microlensing: II. Repeating Events
|
astro-ph
|
In the companion paper we began the task of systematically studying the
detection of planets in wide orbits ($a > 1.5 R_E$) via microlensing surveys.
In this paper we continue, focusing on repeating events. We find that, if all
planetary systems are similar to our own Solar System, reasonable extensions of
the present observing strategies would allow us to detect 3-6 repeating events
per year along the direction to the Bulge. Indeed, if planetary systems with
multiple planets are common, then future monitoring programs which lead to the
discovery of thousands of stellar-lens events will likely discover events in
which several different planets within a single system serve as lenses, with
light curves exhibiting multiple repetitions. In this paper we discuss
observing strategies to maximize the discovery of all wide-orbit planet-lens
events. We also compare the likely detection rates of planets in wide orbits to
those of planets located in the zone for resonant lensing. We find that,
depending on the values of the planet masses and stellar radii of the lensed
sources (which determine whether or not finite source size is important), and
also on the sensitivity of the photometry used by observers, the detection of
planets in wide orbits may be the primary route to the discovery of planets via
microlensing. We also discuss how the combination of resonant and wide-orbit
events can help us to learn about the distribution of planetary system
properties (S 6.1). In addition, by determining the fraction of short-duration
events due to planets, we indirectly derive information about the fraction of
all short-duration events that may be due to low-mass MACHOs (S 6.2).
|
No Label
|
No Label
|
astro-ph/0509678
|
Liese van Zee
|
Oxygen and Nitrogen in Leo A and GR 8
|
astro-ph
|
We present elemental abundances for multiple HII regions in Leo A and GR 8
obtained from long slit optical spectroscopy of these two nearby low luminosity
dwarf irregular galaxies. As expected from their luminosities, and in agreement
with previous observations, the derived oxygen abundances are extremely low in
both galaxies. High signal-to-noise ratio observations of a planetary nebula in
Leo A yield 12 + log(O/H) = 7.30 +/- 0.05; "semi-empirical" calculations of the
oxygen abundance in four HII regions in Leo A indicate 12 + log(O/H) = 7.38 +/-
0.10. These results confirm that Leo A has one of the lowest ISM metal
abundances of known nearby galaxies. Based on results from two HII regions with
high signal-to-noise measurements of the weak [O III] 4363 line, the mean
oxygen abundance of GR 8 is 12 + log(O/H) = 7.65 +/- 0.06; using "empirical"
and "semi-empirical" methods, similar abundances are derived for 6 other GR 8
HII regions. Similar to previous results in other low metallicity galaxies, the
mean log(N/O) = -1.53 +/- 0.09 for Leo A and -1.51 +/- 0.07 for GR 8. There is
no evidence of significant variations in either O/H or N/O in the HII regions.
The metallicity-luminosity relation for nearby (D < 5 Mpc) dwarf irregular
galaxies with measured oxygen abundances has a mean correlation of 12 +
log(O/H) = 5.67 - 0.151 M_B with a dispersion in oxygen about the relationship
of 0.21. These observations confirm that gas-rich low luminosity galaxies have
extremely low elemental abundances in the ionized gas-phase of their
interstellar media. Although Leo A has one of the lowest metal abundances of
known nearby galaxies, detection of tracers of an older stellar population
indicate that it is not a newly formed galaxy as has been proposed for some
other similarly low metallicity star forming galaxies.
|
No Label
|
No Label
|
astro-ph/9908331
|
Rauzy
|
What can we learn from the fluxes of 1.2 Jy IRAS sample?
|
astro-ph
|
We present a new method for fitting peculiar velocity models to complete flux
limited magnitude-redshifts catalogues, using the luminosity function of the
sources as a distance indicator. The method is characterized by its robustness.
In particular, no assumptions are made concerning the spatial distribution of
sources and their luminosity function. Moreover the inclusion of additional
observables, such for example the one carrying the Tully-Fisher information, is
straightforward.
As an illustration of the method, the predicted IRAS peculiar velocity model
is herein tested using the fluxes of the IRAS 1.2 Jy sample as the distance
indicator. The results suggest that this model, while successful in reproducing
locally the cosmic flow, fails to describe the kinematics on larger scales.
|
No Label
|
No Label
|
astro-ph/0510212
|
Victor Tikhomirov
|
How particle collisions increase the rate of accretion from the
cosmological background onto primordial black holes in braneworld cosmology
|
astro-ph
|
It is shown that, contrary to the widespread opinion, particle collisions
considerably increase accretion rate from the cosmological background onto 5D
primordial black holes formed during the high-energy phase of the
Randall-Sundrum Type II braneworld scenario. Increase of accretion rate leads
to much tighter constraints on initial primordial black hole mass fraction
imposed by the critical density limit and measurements of high-energy diffuse
photon background and antiproton excess.
|
No Label
|
No Label
|
astro-ph/9812324
|
Kunth Daniel
|
Gas-Rich Local Dwarf Star-Forming Galaxies and their connection with the
Distant Universe
|
astro-ph
|
I discuss the properties of gas-rich forming galaxies. I particularly
emphasize the latest results on Lyman alpha emission that are relevant to the
search of distant young galaxies. The interdependance of the Lyman alpha escape
with the properties of the ISM in starburst galaxies is outlined. A new model
from G. Tenorio-Tagle and his collaborators explaining Lyman alpha profiles in
starburst galaxies from the hydrodynamics of superbubbles powered by massive
stars is presented. I stress again that since Lyman alpha is primarely a
diagnostic of the ISM, it is mandatory to understand how the ISM and Lyman
alpha are related to firmly relate Lyman alpha to the cosmic star-formation
rate.
|
No Label
|
No Label
|
astro-ph/0501490
|
Lisa Crause
|
V838 Mon: light echo evolution and distance estimate
|
astro-ph
|
Following its 2002 February eruption, V838 Mon developed a light echo that
continues to expand and evolve as light from the outburst scatters off
progressively more distant circumstellar and/or interstellar material.
Multi-filter images of the light echo, obtained with the South African
Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004
December, are analysed and made available electronically. The expansion of the
light echo is measured from the images and the data compared with models for
scattering by a thin sheet and a thin shell of dust. From these model results
we infer that the dust is likely in the form of a thin sheet distant from the
star, suggesting that the material is of interstellar origin, rather than being
from earlier stages in the star's evolution. Although the fit is uncertain, we
derive a stellar distance of ~ 9 kpc and a star-dust distance of ~ 5 pc, in
good agreement with recent results reported from other methods. We also present
JHKL and Cousins UBVRI photometry obtained at the SAAO during the star's
second, third and fourth observing seasons post-outburst. These data show
complex infrared colour behaviour while V838 Mon is slowly brightening in the
optical.
|
No Label
|
No Label
|
astro-ph/0010184
|
Leo Alberto Girardi
|
Theoretical expectations for clump red giants as distance indicators
|
astro-ph
|
Variations of ~0.4 mag are expected in the I-band absolute magnitude of red
clump giants, M_I^RC, as a function of both stellar age and metallicity. This
regardless of some potential theoretical uncertainties. Due to the quite large
differences in mean ages and metallicities of clump stars among galaxies,
systematic changes (also amounting up to ~0.4 mag) come out in their M_I^RC.
These numbers also indicate a distance to the LMC that is not necessarily
``short''.
|
No Label
|
No Label
|
astro-ph/0502010
|
Sheila McBreen
|
Gamma-ray bursts and other sources of giant lightning discharges in
protoplanetary systems
|
astro-ph
|
Lightning in the solar nebula is considered to be one of the probable sources
for producing the chondrules that are found in meteorites. Gamma-ray bursts
(GRBs) provide a large flux of gamma-rays that Compton scatter and create a
charge separation in the gas because the electrons are displaced from the
positive ions. The electric field easily exceeds the breakdown value of ~1 V
m^-1 over distances of order 0.1 AU. The energy in a giant lightning discharge
exceeds a terrestrial lightning flash by a factor of ~10^12. The predicted
post-burst emission of gamma-rays from accretion into the newly formed black
hole or spin-down of the magnetar is sufficiently intense to cause a lightning
storm in the nebula that lasts for days and is more probable than the GRB
because the radiation is beamed into a larger solid angle. The giant outbursts
from nearby soft gamma-ray repeater sources (SGRs) are also capable of causing
giant lightning discharges. The total amount of chondrules produced is in
reasonable agreement with the observations of meteorites. Furthermore in the
case of GRBs most chondrules were produced in a few major melting events by
nearby GRBs and lightning occurred at effectively the same time over the whole
nebula, and provide accurate time markers to the formation of chondrules and
evolution of the solar nebula. This model provides a reasonable explanation for
the delay between the formation of calcium aluminium inclusions (CAIs) and
chondrules.
|
No Label
|
No Label
|
astro-ph/0208426
|
Chris Flynn
|
Simulations of the heating of the Galactic stellar disc
|
astro-ph
|
The velocity dispersion of nearby stars in the Galactic disc is well known to
increase substantially with age; this is the so-called Age-Velocity relation,
and is interpreted as a ``heating'' of the disc as a function of time. We have
studied the heating of the disc due to giant molecular clouds and halo black
holes, using simulations of the orbits of tracer stars embedded in a patch of
the local Galactic disc. We examine a range of masses and number densities of
the giant molecular cloud and halo black hole perturbers. The heating of the
stellar disc in the simulations is fit with a simple power law, and we also fit
this form to the best determinations of Age-Velocity relation as derived from
stars in the solar neighbourhood for which ages can be reliably assigned.
Observationally, the Age-Velocity relation remains poorly constrained and its
determination is probably still dominated by systematic errors. Our simulations
confirm the well known results that there are insufficient GMCs to heat the
Galactic disc appropriately. A range of dark halo black hole scenarios are
verified to heat the stellar disc in the manner expected from analytical
studies, and they reproduce the observed ratio of the stellar velocity
dispersions. Simulations featuring a combination of giant molecular clouds and
halo black holes can explain the observed heating of the stellar disc, but
since other perturbing mechanisms, such as spiral arms, are yet to be
included,we regard this solution as ad hoc.
|
No Label
|
No Label
|
0704.0346
|
Kenji Hamaguchi
|
Diffuse X-ray Emission from the Carina Nebula Observed with Suzaku
|
astro-ph
|
A number of giant HII regions are associated with soft diffuse X-ray
emission. Among these, the Carina nebula possesses the brightest soft diffuse
emission. The required plasma temperature and thermal energy can be produced by
collisions or termination of fast winds from main-sequence or embedded young O
stars, but the extended emission is often observed from regions apart from
massive stellar clusters. The origin of the X-ray emission is unknown.
The XIS CCD camera onboard Suzaku has the best spectral resolution for
extended soft sources so far, and is therefore capable of measuring key
emission lines in the soft band. Suzaku observed the core and the eastern side
of the Carina nebula (Car-D1) in 2005 Aug and 2006 June, respectively. Spectra
of the south part of the core and Car-D1 similarly showed strong L-shell lines
of iron ions and K-shell lines of silicon ions, while in the north of the core
these lines were much weaker. Fitting the spectra with an absorbed thin-thermal
plasma model showed kT~0.2, 0.6 keV and NH~1-2e21 cm-2 with a factor of 2-3
abundance variation in oxygen, magnesium, silicon and iron. The plasma might
originate from an old supernova, or a super shell of multiple supernovae.
|
No Label
|
No Label
|
astro-ph/0202091
|
Naomi Ota
|
X-ray Study of Seventy-nine Distant Clusters of Galaxies: Discovery of
Two Classes of Cluster Size
|
astro-ph
|
We have performed a uniform analysis of 79 clusters of galaxies with the
ROSAT HRI and ASCA to study the X-ray structure and evolution of clusters in
the redshift range 0.1 < z < 1. We determined the average X-ray temperatures
and the bolometric luminosities with ASCA and the spatial distributions of the
X-ray brightness with the ROSAT HRI by utilizing the isothermal beta-model. We
do not find any significant redshift dependence in the X-ray parameters
including the temperature, beta-model parameters, and the central electron
density. Among the parameters, the core radius shows the largest
cluster-to-cluster dispersions. We discovered that the histogram of the core
radius shows two peaks at 60 and 220 kpc. If we divide the cluster samples into
two subgroups corresponding to the two peaks in the core radius distribution,
they show differences in the X-ray and optical morphologies and in the X-ray
luminosity temperature relation. From these observational results, we suggest
that the clusters are divided into at least two subgroups according to the core
radius.
|
No Label
|
No Label
|
astro-ph/9911024
|
Alison Sills
|
Rotation of Horizontal Branch Stars in Globular Clusters
|
astro-ph
|
The rotation of horizontal branch stars places important constraints on
angular momentum evolution in evolved stars and therefore rotational mixing on
the giant branch. Prompted by new observations of rotation rates of horizontal
branch stars, we calculate simple models for the angular momentum evolution of
a globular cluster giant star from the base of the giant branch to the star's
appearance on the horizontal branch. We include mass loss, and infer the
accompanied loss of angular momentum for each of four assumptions about the
internal angular momentum profile. These models are compared to observations of
horizontal branch rotation rates in M13. We find that rapid rotation on the
horizontal branch can be reconciled with slow solid body main sequence rotation
if giant branch stars have differential rotation in their convective envelopes
and a rapidly rotating core, which is then followed by a redistribution of
angular momentum on the horizontal branch. We discuss the physical reasons why
these very different properties relative to the solar case may exist in giants.
Rapid rotation in the core of the main sequence precursors of the rapidly
rotating horizontal branch star, or an angular momentum source on the giant
branch is required for all cases if the rotational velocity of turnoff stars is
less than 4 km s$^{-1}$. We suggest that the observed range in rotation rates
on the horizontal branch is caused by internal angular momentum redistribution
which occurs on a timescale comparable to the evolution of the stars on the
horizontal branch. The apparent lack of rapid horizontal branch rotators hotter
than 12 000 K in M13 could be a consequence of gravitational settling, which
inhibits internal angular momentum transport. Alternative explanations and
observational tests are discussed.
|
No Label
|
No Label
|
0808.2034
|
Sergey Tsygankov
|
X-ray pulsars through the eyes of INTEGRAL
|
astro-ph
|
Recent results of the spectral and timing analysis of X-ray pulsars in hard
X-rays with the INTEGRAL observatory are reviewed. The evolution of the
cyclotron line energy with the source luminosity was studied in detail for the
first time for several sources. It was shown that for V0332+53 this dependence
is linear, but for 4U0115+63 and A0535+262 it is more complicated. There are
some evidences of the "reverse" evolution for GX301-2 and Her X-1, and no
evolution was found for Vela X-1, Cen X-3, etc. A strong dependence of the
pulse fraction on the energy and source luminosity was revealed and studied in
detail. A prominent feature in the pulse fraction dependence on the energy was
discovered near the cyclotron frequency for several bright sources. The
obtained results are compared with results of observations in standard X-rays
and briefly discussed in terms of current models; some preliminary explanations
are proposed.
|
No Label
|
No Label
|
0707.0897
|
Julian North
|
Orbital Solution & Fundamental Parameters of sigma Scorpii
|
astro-ph
|
The first orbital solution for the spectroscopic pair in the multiple star
system sigma Scorpii, determined from measurements with the Sydney University
Stellar Interferometer (SUSI), is presented. The primary component is of beta
Cephei variable type and has been one of the most intensively studied examples
of its class. The orbital solution, when combined with radial velocity results
found in the literature, yields a distance of 174(+23,-18) pc, which is
consistent with, but more accurate than the Hipparcos value. For the primary
component we determine 18.4+/-5.4 M_sun, -4.12+/-0.34 mag and 12.7+/-1.8 R_sun
for the mass, absolute visual magnitude and radius respectively. A B1 dwarf
spectral type and luminosity class for the secondary is proposed from the mass
determination of 11.9+/-3.1 M_sun and the estimated system age of 10 Myr.
|
No Label
|
No Label
|
astro-ph/9412004
|
C. Bird
|
Subcluster Merger and Galaxy Infall in A2151
|
astro-ph
|
We have obtained a 12.5 ksec image of the Hercules Cluster, A2151, with the
{\it ROSAT} PSPC. Comparison of the optical and X-ray data suggest the presence
of at least three distinct subclusters in A2151. The brightest X-ray emission
coincides with the highest-density peak in the galaxy distribution, and is
bimodal. The northern subclump, distinct in position and velocity, has {\it no}
detectable X-ray gas. The eastern subclump, apparent in the optical contour
map, is indistinguishable from the main clump in velocity space but is clearly
visible in the X-ray image. X-ray spectra derived from the central peak of
emission yield a best-fit temperature of 1.6 keV. The emission coincident with
the eastern clump of galaxies is cooler, 0.8 keV, and is outside the 90\%
confidence intervals of the central peak temperature. We suggest that the
eastern and central subclusters have recently undergone a merger event. The
lack of X-ray emission to the north suggests that those galaxies do not form a
physically-distinct structure (i.e. they are not located within a distinct
gravitational potential), but rather that they are falling into the cluster
core along the filament defined by the Hercules Supercluster.
|
No Label
|
No Label
|
astro-ph/0104124
|
Rose A. Finn
|
WFPC2 Imaging of Quasar Environments: A Comparison of LBQS and HST
Archive Quasars
|
astro-ph
|
We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2)
data on the large-scale environments of 16 0.39 < z < 0.51 quasars from the
Large Bright Quasar Survey (LBQS). The LBQS quasars are representative of the
radio-quiet population, and this is the first look at their large-scale
environments. We compare the LBQS environments with the environments of 27 0.15
< z < 0.55 quasars selected from the HST Archive. The majority of the Archive
quasars are from the PG and PKS surveys, and these quasars are more luminous on
average than the LBQS. By comparing the LBQS and Archive environments, we
investigate whether previous quasar environment studies have been biased due to
studying unusually radio or optically luminous quasars. We detect a significant
excess of galaxies around the Archive quasars but find no such excess around
the LBQS quasars. We calculate the amplitude of the spatial correlation
function and find that the LBQS environments are consistent with that of the
typical galaxy while the Archive environments are slightly less rich than Abell
0 clusters. We find no difference between the environments of radio-loud and
radio-quiet quasars in either sample. However, comparison with previously
published work shows that the LBQS radio-loud quasars are in sparse
environments when compared with other radio-loud quasars, and the Archive
radio-quiet quasars are in dense environments compared to other radio-quiet
quasars. The richer environments of the Archive radio-quiet quasars can not be
explained by their higher optical luminosities.We find a positive correlation
(95%) between radio luminosity and environment for the radio-loud quasars. This
may explain why the LBQS radio-loud quasars, which are less radio luminous, are
in sparser environments.
|
No Label
|
No Label
|
astro-ph/0305565
|
Edwin A. Bergin
|
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical
Equilibrium of T Tauri Disks
|
astro-ph
|
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.
|
No Label
|
No Label
|
0812.2358
|
Nicolas Lodieu
|
Identifying nearby field T dwarfs in the UKIDSS Galactic Clusters Survey
|
astro-ph
|
We present the discovery of two new late-T dwarfs identified in the UKIRT
Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 2
(DR2). These T dwarfs are nearby old T dwarfs along the line of sight to
star-forming regions and open clusters targeted by the UKIDSS GCS. They are
found towards the Alpha Per cluster and Orion complex, respectively, from a
search in 54 square degrees surveyed in five filters. Photometric candidates
were picked up in two-colour diagrams, in a very similar manner to candidates
extracted from the UKIDSS Large Area Survey (LAS) but taking advantage of the Z
filter employed by the GCS. Both candidates exhibit near-infrared J-band
spectra with strong methane and water absorption bands characteristic of late-T
dwarfs. We derive spectral types of T6.5+/-0.5 and T7+/-1 and estimate
photometric distances less than 50 pc for UGCS J030013.86+490142.5 and UGCS
J053022.52-052447.4, respectively. The space density of T dwarfs found in the
GCS seems consistent with discoveries in the larger areal coverage of the
UKIDSS Large Area Survey, indicating one T dwarf in 6-11 square degrees. The
final area surveyed by the GCS, 1000 square degrees in five passbands, will
allow expansion of the LAS search area by 25%, increase the probability of
finding ultracool brown dwarfs, and provide optimal estimates of contamination
by old field brown dwarfs in deep surveys to identify such objects in open
clusters and star-forming regions.
|
No Label
|
No Label
|
astro-ph/0302399
|
Rucinski
|
Radial Velocity Studies of Close Binary Stars.VIII
|
astro-ph
|
Radial-velocity measurements and sine-curve fits to the orbital velocity
variations are presented for the seventh set of ten close binary systems: V410
Aur, V523 Cas, QW Gem, V921 Her, V2357 Oph, V1130 Tau, HN UMa, HX UMa, HD
93917, NSV 223. All systems, but three (V523 Cas, HD 93917, NSV 223), were
discovered photometrically by the Hipparcos mission. All systems are
double-lined (SB2) binaries and all, but the detached, very close system V1130
Tau, are contact binaries. The broadening-function permitted improvement of the
orbital elements for V523 Cas, which was the only system observed before for
radial velocity variations. Spectroscopic/visual companions were detected for
V410 Aur and HX UMa.
|
No Label
|
No Label
|
astro-ph/9711330
|
Andrew Reid
|
Radio Study of X-ray Clusters I: A3528 --- a pre-merging cluster in the
core of the Shapley Supercluster
|
astro-ph
|
As part of an extensive radio--IR--optical--X-ray study of ROSAT clusters of
galaxies in the Hydra region we have observed the bimodal Abell cluster A3528,
located in the core of the Shapley Supercluster (z ~ 0.053), with the Molonglo
Observatory Synthesis Telescope at 843 MHz and the Australia Telescope Compact
Array at 1.4 and 2.4 GHz. This is part I in a series of papers which look at
the relationship between the radio and X-ray emission in samples of ROSAT
selected clusters.
The radio source characteristics --- tailed morphologies and steep spectra
--- are consistent with the effects of a dense intra-cluster medium and the
pre-merging environment of A3528. In particular, we present evidence that the
minor member of the radio loud dumbbell galaxy located at the centre of the
northern component of A3528 is on a plunging orbit. We speculate that this
orbit may have been induced by the tidal interactions between the merging
components of A3528. In addition, the radio source associated with the dominant
member of the dumbbell galaxy exhibits many of the characteristics of Compact
Steep Spectrum sources. We argue that the radio emission from this source was
triggered 10^6 yr ago by tidal interactions between the two members of the
dumbbell galaxy, strengthening the argument that CSS sources are young.
Re-analysis of the archive pointed PSPC data using multi-resolution filtering
suggests the presence of an AGN and/or a cooling flow in the southern component
of A3528.
|
No Label
|
No Label
|
astro-ph/9906311
|
Andrew Blain
|
Dust-obscured star formation and AGN fuelling in hierarchical models of
galaxy evolution
|
astro-ph
|
A large fraction of the luminous distant submm-wave galaxies recently
detected using SCUBA on the JCMT appear to be associated with interacting
optical counterparts. We investigate these systems using a simple hierarchical
clustering model of galaxy evolution, in which the large luminosity of the
SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a
burst of either star formation activity or the fuelling of an active galactic
nucleus (AGN). The models are well constrained by the observed spectrum of the
FIR/submm background radiation and the 60-micron counts of low-z IRAS galaxies.
The ratio between the total amount of energy released during mergers and the
mass of dark matter involved must increase sharply with z at z<1, and then
decrease at higher z, independent of the fraction of the luminosity of mergers
that is produced by starbursts and AGN. One additional parameter - the
reciprocal of the product of the duration of the enhanced luminosity produced
by the merger and the fraction of mergers that induce an enhanced luminosity,
which we call the activity parameter - is introduced, to allow the relationship
between merging dark matter haloes and the observed counts of distant dusty
galaxies to be investigated. The observed counts can only be reproduced if the
activity parameter is greater by a factor of about 5 and 100 at redshifts of 1
and 3 respectively, compared with the present epoch. Hence, if mergers account
for the SCUBA galaxies, then the merger process must have been much more
violent at high redshifts. We discuss the counts of galaxies and the intensity
of background radiation in the optical/near-IR wavebands in the context of
these hierarchical models, and thus investigate the relationship between the
populations of SCUBA and Lyman-break galaxies.
|
No Label
|
No Label
|
astro-ph/0605069
|
Henry A. (Chip) Kobulnicky
|
A New Look at the Binary Characteristics of Massive Stars
|
astro-ph
|
We constrain the properties of massive binaries by comparing radial velocity
data of Cygnus OB2 with Monte Carlo models. Our comparisons test several
popular prescriptions for massive binary parameters. We explore a range of true
binary fraction, F, a range of power-law slopes, \alpha, describing the
distribution of companion masses, and a range of power-law slopes, \beta,
describing the distribution of orbital separations. We also consider
distributions of secondary masses described by a Miller-Scalo type initial mass
function and by a two-component IMF that includes a substantial ``twin''
population with M_2 ~ M_1. We show that binary fractions F<0.7 are considerably
less probable than F>0.8. Thus, the true binary fraction is high. For F=1.0 and
a distribution of orbital separations near the canonical Opik's Law
distribution (i.e., flat; \beta=0), the power law slope of the mass ratio
distribution is \alpha= -0.6 - 0.0. For F~0.8, \alpha is somewhat larger, in
the range -0.4 - 1.0. In any case, the secondary star mass function is
inconsistent with a Miller-Scalo -like IMF unless the lower end is truncated
below ~ 2--4 solar masses. In other words, massive stars preferentially have
massive companions. The best fitting models are described by a Salpeter or
Miller-Scalo IMF for 60% of secondary star masses with the other 40% of
secondaries having M_2 ~ M_1, i.e., ``twins''. These best-fitting model
parameters simultaneously predict the fraction of type Ib/c supernovae to be
30-40% of all core-collapse supernovae, in agreement with recent observational
estimates.
|
No Label
|
No Label
|
0810.0284
|
Douglas Braun
|
Prospects for the Detection of the Deep Solar Meridional Circulation
|
astro-ph
|
We perform helioseismic holography to assess the noise in p-mode travel-time
shifts which would form the basis of inferences of large-scale flows throughout
the solar convection zone. We also derive the expected travel times from a
parameterized return (equatorward) flow component of the meridional circulation
at the base of the convection zone from forward models under the assumption of
the ray and Born approximations. From estimates of the signal-to-noise ratio
for measurements focused near the base of the convection zone, we conclude that
the helioseismic detection of the deep meridional flow including the return
component may not be possible using data spanning an interval less than a solar
cycle.
|
No Label
|
No Label
|
astro-ph/0612071
|
Slawomir Stachniewicz
|
Collapse of the Primordial Gas Clouds in the Presence of UV Radiation
Field
|
astro-ph
|
Our goal is to study the effects of the UV radiation from the first stars,
quasars and hypothetical Super Heavy Dark Matter (SHDM) particle decays on the
formation of primordial bound objects in the Universe. We trace the evolution
of a spherically symmetric density perturbation in the Lambda Cold Dark Matter
and MOND model, solving the frequency-dependent radiative transfer equation,
non-equilibrium chemistry, and one-dimensional gas hydrodynamics. We
concentrate on the destruction and formation processes of the $H_{2}$ molecule,
which is the main coolant in the primordial objects.
|
No Label
|
No Label
|
astro-ph/9311020
|
Ray Carlberg
|
Mapping Moderate Redshift Cluster
|
astro-ph
|
To test whether clusters have rising mass to light ratios at large radii and
to estimate the amplitude of the density fluctuation spectrum on the scale of
10\hmpc\ the Canadian Network for Observational Cosmology (CNOC) cluster
collaboration is obtaining Multi-Object Spectrograph velocities and two colour
photometry for a sample of $\simeq$1000 cluster galaxies and $\simeq$2000 field
galaxies in a 5\hmpc\ neighborhood of high X-ray luminosity clusters at
$z\simeq 0.3$. X-ray selection of the cluster sample picks out objects on the
basis of the depth of their potential well depth, and is insensitive to both
projection effects and galaxy biases. The galaxy dataset, with selection
controlled using automated photometry, automated mask design, spectral
selection modeling, and accurate velocities, will be the best available at any
redshift for our tests. Measuring mass-to-light ratios (M/L) at large radii
requires a statistical removal of field galaxies projected into the redshift
space of the cluster. This can be done relatively accurately for moderate
redshift clusters, using the large number of foreground and background
galaxies. The CNOC sample also provides a sensitive test of the density
fluctuation spectrum on cluster scales. The 30 or so redshifts available in
each of three clusters gives an average velocity dispersion of about 1000 \kms
which indicates that the $\sigma_8$ normalization parameter is in the range of
$0.6\lta \sigma_8\lta 0.9$.
|
No Label
|
No Label
|
astro-ph/9906188
|
Letizia Stanghellini
|
Morphology and Evolution of Galactic and Magellanic Cloud Planetary
Nebulae
|
astro-ph
|
Planetary nebulae (PNe) exist in a range of different morphologies, from very
simple and symmetric round shells, to elliptical, bipolar, and even quadrupolar
shapes. They present extremely complex ensembles of filaments, knots, ansae,
and shell multiplicity. It is then overwhelmingly complicated to derive
reasonable evolutionary paths to justify the observed shapes of PNe. The
confrontation between the evolution of the shells and that of the central stars
is needed to understand the origin of the morphological variety. We present
some background and recent results on the correlations between PN morphology
and PN nuclei (PNNi) evolution, including a study on the Magellanic Cloud PNe.
|
No Label
|
No Label
|
astro-ph/0001514
|
Shlosman
|
Dynamics of Warm-Absorbing Gas in Seyfert Galaxies: NGC 5548
|
astro-ph
|
A hydromagnetic (MHD) wind from a clumpy molecular accretion disk is invoked
to explain observations of warm absorbing (WA) gas in UVX from Sy galaxies.
This paper focuses on two issues: (1) compatibility of kinematics and dynamics
of MHD wind with the observed properties of WAs; and (2) relationship between
the UVX absorptions. We provide an in-depth comparison between the MHD model
and the Sy 1 galaxy NGC 5548, which at high spectral resolution exhibits a
number of discrete UV absorption components. We find that: (1) the total column
densities of Ovii, Oviii and H, are reproduced by constraining the UV ion
column densities of Civ and Nv in each component to lie within a factor of 2 of
their observed values and optimizing over the possible sets of component
ionization states and Civ column densities; (2) the WA exists in the outer part
of the wind and is not a continuation of the flow in the BLR; and (3) the WA
extends in radial and polar directions and is ionization-stratified. X-ray
absorption is found to be heavily biased towards smaller r, and UV absorption
originates at larger distances from the central continuum source. We show that
the discrete absorption components along the line-of-sight are intrinsically
clumpy. Density differences between kinematic components result in a range of
ionization and recombination timescales. We further test the applicability of
the MHD wind to WAs in general, by constructing a quasi-continuous flow model,
and extending it to arbitrary aspect angles. We estimate the fraction of Sy 1s
having detectable WAs with larger Ovii column density than Oviii, and the range
of total H column densities. We also find that the ratio of Ovii to Oviii
optical depths can serve as a new diagnostic of AGN aspect angle.
|
No Label
|
No Label
|
astro-ph/0606479
|
Fabien Daniel
|
The Excitation of N$_2$H$^+$ in Interstellar Molecular Clouds. I -
Models
|
astro-ph
|
We present LVG and non-local radiative transfer calculations involving the
rotational and hyperfine structure of the spectrum of N$_2$H$^+$ with
collisional rate coefficients recently derived by us. The goal of this study is
to check the validity of the assumptions made to treat the hyperfine structure
and to study the physical mechanisms leading to the observed hyperfine
anomalies.
We find that the usual hypothesis of identical excitation temperatures for
all hyperfine components of the $J$=1-0 transition is not correct within the
range of densities existing in cold dense cores, i.e., a few 10$^4$ $\textless$
n(H$_2$) $\textless$ a few 10$^6$ cm$^{-3}$. This is due to different radiative
trapping effects in the hyperfine components. Moreover, within this range of
densities and considering the typical abundance of N$_2$H$^+$, the total
opacity of rotational lines has to be derived taking into account the hyperfine
structure. The error made when only considering the rotational energy structure
can be as large as 100%. Using non-local models we find that, due to
saturation, hyperfine anomalies appear as soon as the total opacity of the
$J$=1-0 transition becomes larger than $\simeq$ 20. Radiative scattering in
less dense regions enhance these anomalies, and particularly, induce a
differential increase of the excitation temperatures of the hyperfine
components. This process is more effective for the transitions with the highest
opacities for which emerging intensities are also reduced by self-absorption
effects. These effects are not as critical as in HCO$^+$ or HCN, but should be
taken into account when interpreting the spatial extent of the N$_2$H$^+$
emission in dark clouds.
|
No Label
|
No Label
|
astro-ph/0012497
|
Simon Prunet
|
Error estimation for the MAP experiment
|
astro-ph
|
We report here the first full sky component separation and CMB power spectrum
estimation using a Wiener filtering technique on simulated data from the
upcoming MAP experiment, set to launch in early 2001. The simulations included
contributions from the three dominant astrophysical components expected in the
five MAP spectral bands, namely CMB radiation, Galactic dust, and synchrotron
emission. We assumed a simple homogeneous and isotropic white noise model and
performed our analysis up to a spherical harmonic multipole lmax=512 on the
fraction of the sky defined by b>20 degrees. We find that the reconstruction
errors are reasonably well fitted by a Gaussian with an rms of 24 $\mu $K, but
with significant deviations in the tails. Our results further support the
predictions on the resulting CMB power spectrum of a previous estimate by
Bouchet and Gispert (1999), which entailed a number of assumptions this work
removes.
|
No Label
|
No Label
|
astro-ph/9904336
|
Dipanjan Mitra
|
Revisiting the Shape of Pulsar Beams
|
astro-ph
|
Characterizing the shape and evolution of pulsar radio emission beams is
important for understanding the observed emission. The various attempts by
earlier workers investigating beam shapes have resulted in widely different
conclusions. Using a carefully selected subset of the recently published
multifrequency polarimetry observations of 300 radio pulsars (Gould & Lyne,
1998), we attempt to model the shape of pulsar beams. Assuming that the beam
shape is elliptical, in general, and that it may depend on the angle between
the rotation and the magnetic axes, we seek a consistent model where we also
solve for the dependence of the beam size on frequency. From the six-frequency
data on conal triple and multiple component profiles, we show that a) the
pulsar emission beams follow a nested cone structure with at least three
distinct cones, although only one or more of the cones may be active in a given
pulsar; b) each emission cone is illuminated in the form of an annular ring of
width typically about 20% of the cone radius.
Although some slight preference is evident for a model where the beam is
circular for an aligned rotator & latitudinally compressed for an orthogonal
rotator, the possibility that the beam shape is circular at all inclinations is
found to be equally consistent with the data. While the overall size scales as
$P^{-0.5}$ (where $P$ is the pulsar period) as expected from the notion of
dipolar open field lines, we see no evidence in support of the beam shape
evolution with pulsar period.
|
No Label
|
No Label
|
astro-ph/0608193
|
Chris Flynn
|
On the mass-to-light ratio of the local Galactic disc and the optical
luminosity of the Galaxy
|
astro-ph
|
We measure the volume luminosity density and surface luminosity density
generated by the Galactic disc, using accurate data on the local luminosity
function and the disc's vertical structure. From the well measured volume mass
density and surface mass density, we derive local volume and surface
mass-to-light ratios for the Galactic disc, in the bands B, V and I. We obtain
mass-to-light ratios for the local column of stellar matter of (M/L)_B = 1.4
+/- 0.2, (M/L)_V = 1.5 +/- 0.2 and (M/L)_I = 1.2 +/- 0.2. The dominant
contributors to the surface luminosity in these bands are main sequence
turn-off stars and giants. Our results on the colours and mass-to-light ratios
for the ``Solar cylinder'' well agree with population synthesis predictions
using Initial Mass Functions typical of the Solar Neighbourhood. Finally we
infer the global luminosity of the Milky Way, which appears to be
under-luminous by about 1-sigma with respect to the main locus of the
Tully-Fisher relation, as observed for external galaxies.
|
No Label
|
No Label
|
astro-ph/0209508
|
Wayne Hayes
|
Shadowing high-dimensional Hamiltonian systems: the gravitational n-body
problem
|
astro-ph
|
A {\it shadow} is an exact solution to a chaotic system of equations that
remains close to a numerically computed solution for a long time, ending in a
{\it glitch}. We study the distribution of shadow durations at low dimension
and how shadow durations scale as dimension increases up to 300 in a slightly
simplified gravitational n-body system. We find that ``softened'' systems are
shadowable for many tens of crossing times even for large n, while in an
``unsoftened'' system each particle encounters glitches independently as a
Poisson process, giving shadow durations that scale as 1/n.
|
No Label
|
No Label
|
astro-ph/0401074
|
Eli Dwek
|
The Detection of Cold Dust in Cas A: Evidence for the Formation of
Metallic Needles in the Ejecta
|
astro-ph
|
Recently, Dunne et al. (2003) obtained 450 and 850 micron SCUBA images of
CasA, and reported the detection of 2-4 M_sun of cold, 18K, dust in the
remnant. Here we show that their interpretation of the observations faces
serious difficulties. Their inferred dust mass is larger than the mass of
refractory material in the ejecta of a 10 to 30 M_sun star. The cold dust model
faces even more difficulties if the 170 micron observations of the remnant are
included in the analysis, decreasing the cold dust temperature to ~ 8K, and
increasing its mass to > 20 M_sun. We offer here a more plausible
interpretation of their observation, in which the cold dust emission is
generated by conducting needles with properties that are completely determined
by the combined submillimeter and X-ray observations of the remnant. The
needles consist of metallic whiskers with <1% of embedded impurities that may
have condensed out of blobs of material that were expelled at high velocities
from the inner metal-rich layers of the star in an asymmetric explosion. The
needles are collisionally heated by the shocked gas to a temperature of 8K.
Taking the destruction of needles into account, a dust mass of only 1E-4 to
1E-3M_sun is needed to account for the observed SCUBA emission. Aligned in the
magnetic field, needles may give rise to observable polarized emission. The
detection of submillimeter polarization will therefore offer definitive proof
for a needle origin for the cold dust emission. Supernovae may still be proven
to be important sources of interstellar dust, but the evidence is still
inconclusive.
|
No Label
|
No Label
|
astro-ph/0208214
|
Brian E. Wood
|
Elemental Abundances and Ionization States within the Local Interstellar
Cloud Derived from HST and FUSE Observations of the Capella Line of Sight
|
astro-ph
|
We use ultraviolet spectra of Capella from the Hubble Space Telescope (HST)
and Far Ultraviolet Spectroscopic Explorer (FUSE) satellites to study
interstellar absorption lines from the Local Interstellar Cloud (LIC).
Measurements of these lines are used to empirically determine the ionization
states of carbon, nitrogen, and silicon in the LIC, for comparison with the
predictions of theoretical photoionization models. We find that the observed
ionization states are consistent with previously published photoionization
predictions. Total abundances are determined for the elements mentioned above,
and others, for comparison with solar abundances. Magnesium, aluminum, silicon,
and iron are all depleted by at least a factor of 10 toward Capella. The
abundances of carbon, nitrogen, and oxygen are essentially solar, although the
error bars are large enough to also allow depletions of about a factor of 2 for
these elements.
|
No Label
|
No Label
|
astro-ph/0605002
|
Marek Jamrozy
|
The new sample of giant radio sources II. Update of optical
counterparts, further spectroscopy of identified faint host galaxies,
high-frequency radio maps, and polarisation properties of the sources
|
astro-ph
|
Our sample of giant radio-source candidates, published in Paper I (Machalski
et al. 2001), is updated and supplemented with further radio and optical data.
In this paper we present: (i) newly detected host galaxies, their photometric
magnitude, and redshift estimate for the sample sources not identified yet,
(ii) optical spectra and spectroscopic redshift for the host galaxies fainter
than about 18.5 mag taken with the Apache Point Observatory 3.5m telescope, and
(iii) the VLA 4.9 GHz total-intensity and polarised-intensity radio maps of the
sample members. In a few cases they reveal extremely faint radio cores
undetected before, which confirm the previously uncertain optical
identifications. The radio maps are analysed and the polarisation properties of
the sample sources summarised. A comparison of our updated sample with three
samples published by other authors implies that all these four samples probe
the same part of the population of extragalactic radio sources. There is no
significant difference between the distributions of intrinsic size and radio
power among these samples. The median redshift of 0.38 +/- 0.07 in our sample
is the highest among the corresponding values in the four samples, indicating
that the angular size and flux-density limits in our sample, lower than those
for the other three samples, result in effective detections of more distant,
giant-size galaxies compared to those detected in the other samples. This
sample and a comparison sample of `normal'-size radio galaxies will be used in
Paper III (Machalski & Jamrozy 2006) to investigate of a number of trends and
correlations in the entire data.
|
No Label
|
No Label
|
astro-ph/9906453
|
Denise Rocha Goncalves
|
Does magnetic pressure affect the ICM dynamics?
|
astro-ph
|
A possible discrepancy found in the determination of mass from gravitational
lensing data, and from X-rays observations, has been largely discussed in the
latest years (for instance, Miralda-Escude & Babul (1995)). Another important
discrepancy related to these data is that the dark matter is more centrally
condensed than the X-ray-emitting gas, and also with respect to the galaxy
distribution (Eyles et al. 1991). Could these discrepancies be consequence of
the standard description of the ICM, in which it is assumed hydrostatic
equilibrium maintained by thermal pressure? We follow the evolution of the ICM,
considering a term of magnetic pressure, aiming at answering the question
whether or not these discrepancies can be explained via non-thermal terms of
pressure. Our results suggest that the magnetic pressure could only affect the
dynamics of the ICM on scales as small as < 1kpc. Our models are constrained by
the observations of large and small scale fields and we are successful at
reproducing available data, for both Faraday rotation limits and inverse
Compton limits for the magnetic fields. In our calculations the radius (from
the cluster center) in which magnetic pressure reaches equipartition is smaller
than radii derived in previous works, as a consequence of the more realistic
treatment of the magnetic field geometry and the consideration of a sink term
in the cooling flow.
|
No Label
|
No Label
|
astro-ph/0011023
|
Peter Hoeflich
|
Jet Induced Supernovae-Hydrodynamics and Observational Consequences
|
astro-ph
|
Core collapse supernovae (SN) are the final stages of stellar evolution in
massive stars during which the central region collapses, forms a neutron star
(NS), and the outer layers are ejected. Recent explosion scenarios assumed that
the ejection is due to energy deposition by neutrinos into the envelope but
detailed models do not produce powerful explosions. There is mounting evidence
for an asphericity in the SN which is difficult to explain within this picture.
This evidence includes the observed high polarization, pulsar kicks, high
velocity iron-group and intermediate-mass elements material observed in
remnants, etc.
The discovery of highly magnetars revived the idea that the basic mechanism
for the ejection of the envelope is related to a highly focused MHD-jet formed
at the NS. Our 3-D hydro simulations of the jet propagation through the star
confirmed that the mechanism can explain the asphericities.
In this paper, detailed 3-D models for jet induced explosions of "classical"
core collapse supernovae are presented. We demonstrate the influence of the jet
properties and of the underlaying progenitor structure on the final density and
chemical structure. Finally, we discuss the observational consequences,
predictions and tests of this scenario.
|
No Label
|
No Label
|
astro-ph/0601094
|
Keiichi Wada
|
High-Resolution Simulations of a Moon-Forming Impact and Post-Impact
Evolution
|
astro-ph
|
In order to examine the ``giant impact hypothesis'' for the Moon formation,
we run the first grid-based, high-resolution hydrodynamic simulations for an
impact between proto-Earth and a proto-planet. The spatial resolution for the
impact-generated disk is greatly improved from previous particle-based
simulations. This allows us to explore fine structures of a circumterrestrial
debris disk and its long-term evolution. We find that in order to form a debris
disk from which a lunar-sized satellite can be accumulated, the impact must
result in a disk of mostly liquid or solid debris, where pressure is not
effective, well before the accumulation process starts. If the debris is
dominated by vapor gas, strong spiral shocks are generated, and therefore the
circumterrestrial disk cannot survive more than several days. This suggests
that there could be an appropriate mass range for terrestrial planets to harbor
a large moon as a result of giant impacts, since vaporization during an impact
depends on the impact energy.
|
No Label
|
No Label
|
0804.3578
|
Keitaro Takahashi
|
Behavior of magnetic fields around the cosmic recombination
|
astro-ph
|
Several mechanisms have been proposed to generate primordial magnetic fields
and it is often assumed that magnetic fields are not affected by a sharp drop
in ionization rate due to the cosmic recombination. We investigate the validity
of the assumption by studying the behavior of magnetic fields and fluid motion
around recombination. Fluid equations including the effect of recombination are
considered for protons, electrons and neutral hydrogens separately, combining
Maxwell equations. We find that the residual ionization rate required for the
conservation of magnetic field at cosmological scales is about $10^{-10}$,
which is much smaller than the standard value $\sim 10^{-4}$. Further we will
show the acceleration of protons and electrons in the process of recombination
which conpensates the decrease in the carrier of electric current in order to
preserve electric current and then magnetic fields.
|
No Label
|
No Label
|
astro-ph/0110587
|
Jose M. Diego Rodriguez
|
A Bayesian non-parametric method to detect clusters in Planck data
|
astro-ph
|
We show how one may expect a significant number of SZ detections in future
Planck data without any of the typical assumptions needed in present component
separation methods, such as about the power spectrum or the frequency
dependence of any of the components, circular symmetry or a typical scale for
the clusters. We reduce the background by subtracting an estimate of the point
sources, dust and CMB. The final SZE map is estimated in Fourier space. The
catalogue of returned clusters is complete above flux 200 mJy (353 GHz) while
the lowest flux reached by our method is about 70 mJy (353 GHz). We predict a
large number of detections (about 9000) in 4/5 of the sky. This large number of
SZ detections will allow a robust and consistent analysis of the evolution of
the cluster population with redshift and will have important implications for
determining the best cosmological model.
|
No Label
|
No Label
|
0807.1180
|
Jarrod R. Hurley
|
Self-consistent simulations of star cluster formation from gas clouds
under the influence of galaxy-scale tidal fields
|
astro-ph
|
We present the first results of a project aimed at following the formation
and long-term dynamical evolution of star clusters within the potential of a
host galaxy. Here we focus on a model evolved within a simplified potential
representing the Large Magellanic Cloud. This demonstrates for the first time
the self-consistent formation of a bound star cluster from a giant molecular
cloud. The model cluster reproduces the density profiles and structural
characteristics of observed star clusters.
|
No Label
|
No Label
|
astro-ph/9709087
|
Gabriela Surpi
|
Weak-Lensing by Large-Scale Structure and the Polarization Properties of
Distant Radio-Sources
|
astro-ph
|
We estimate the effects of weak lensing by large-scale density
inhomogeneities and long-wavelength gravitational waves upon the polarization
properties of electromagnetic radiation as it propagates from cosmologically
distant sources. Scalar (density) fluctuations do not rotate neither the plane
of polarization of the electromagnetic radiation nor the source image. They
produce, however, an appreciable shear, which distorts the image shape, leading
to an apparent rotation of the image orientation relative to its plane of
polarization. In sources with large ellipticity the apparent rotation is rather
small, of the order (in radians) of the dimensionless shear. The effect is
larger at smaller source eccentricity. A shear of 1% can induce apparent
rotations of around 5 degrees in radio sources with the smallest eccentricity
among those with a significant degree of integrated linear polarization. We
discuss the possibility that weak lensing by shear with rms value around or
below 5% may be the cause for the dispersion in the direction of integrated
linear polarization of cosmologically distant radio sources away from the
perpendicular to their major axis, as expected from models for their magnetic
fields. A rms shear larger than 5% would be incompatible with the observed
correlation between polarization properties and source orientation in distant
radio galaxies and quasars. Gravity waves do rotate both the plane of
polarization as well as the source image. Their weak lensing effects, however,
are negligible.
|
No Label
|
No Label
|
astro-ph/0612499
|
D. M. Alexander
|
Weighing Black Holes in high-z SCUBA Galaxies
|
astro-ph
|
Deep SCUBA surveys have uncovered a population of dust-enshrouded
star-forming galaxies at z~2. Using the ultra-deep 2 Ms Chandra Deep
Field-North survey we recently showed that a large fraction of these systems
are also undergoing intense black-hole growth. Here we provide further
constraints on the properties of the black holes in SCUBA galaxies using the
virial black-hole mass estimator. We show that typical SCUBA galaxies are
likely to host black holes with M_BH~10^7-10^8 M_solar which are accreting at,
or close to, the Eddington limit. These results provide qualitative support for
our earlier conclusion that the growth of the black hole lags that of the host
galaxy in these massive ultraluminous galaxies.
|
No Label
|
No Label
|
astro-ph/9908041
|
Krzysztof Stanek
|
UBVI Color-Magnitude Diagrams in Baade's Window: Metallicity Range,
Implications for the Red Clump Method, Color ``Anomaly'' and the Distances to
the Galactic Center and the Large Magellanic Cloud
|
astro-ph
|
(Abridged) We analyze the UBVI color-magnitude diagrams towards the Galactic
bulge in a relatively low-reddening region of Baade's Window. The dereddened
red giant branch is very wide [~1.0 mag in (U-B)_0 and ~0.4 mag in (B-V)_0 and
(V-I)_0], indicating a significant dispersion of stellar metallicities, which
by comparison with the theoretical isochrones and data for Galactic clusters we
estimate to lie between approximately -0.7< [Fe/H] < +0.3. We also discuss the
metallicity dependence of the red clump I-band brightness and we show that it
is between 0.1-0.2 mag dex^{-1}. This agrees well with the previous empirical
determinations and the models of stellar evolution.
The de-reddened (V-I)_0 color of the red clump in the observed bulge field is
<(V-I)_0> = 1.066, sigma_{(V-I)_0} = 0.14, i.e. 0.056 mag redder than the local
stars with good parallaxes measured by Hipparcos. When we use our data to
re-derive the red clump distance to the Galactic center, we obtain the
Galactocentric distance modulus \mu_{0,GC}=14.69 +- 0.1 mag (R_0=8.67 +- 0.4
kpc), with error dominated by the systematics of photometric calibration. We
then discuss the systematics of the red clump method and how they affect the
red clump distance to the Large Magellanic Cloud. We argue that the value of
distance modulus \mu_{0,LMC}=18.24 +- 0.08 (44.5 +- 1.7 kpc), recently refined
by Udalski, is currently the most secure and robust of all LMC distance
estimates. This has the effect of increasing any LMC-tied Hubble constant by
about 12%, including the recent determinations by the HST Key Project and
Sandage et al.
The UBVI photometry is available through the anonymous ftp service.
|
No Label
|
No Label
|
astro-ph/0511407
|
Marijke Haverkorn
|
Magnetic fields in the Southern Galactic Plane Survey
|
astro-ph
|
The Southern Galactic Plane Survey (SGPS) is a 1.4 GHz radio polarization and
HI survey in a large part of the inner Galactic plane at a resolution of about
an arcmin. Depolarization and Faraday rotation of polarized radiation from
diffuse Galactic synchrotron emission, pulsars, and extragalactic sources can
be used to infer information about the strength and structure of the Galactic
magnetic field. Here, we discuss science results of the polarization data from
the SGPS. We show from statistical analysis of rotation measures of polarized
extragalactic sources that fluctuations in the magneto-ionized medium of the
spiral arms are probably mainly caused by HII regions, while the rotation
measure fluctuations in the interarm regions may be connected to the
interstellar turbulent cascade. Furthermore, the variations of rotation measure
with Galactic longitude enable modeling of the large-scale component of the
Galactic magnetic field, including determination of the number and location of
magnetic field reversals. Finally, the SGPS is an excellent way to study
subparsec-scale structure in the ionized ISM by way of depolarization studies
in HII regions.
|
No Label
|
No Label
|
astro-ph/0103061
|
Julian Krolik
|
Magnetic Stresses at the Inner Edges of Accretion Disks Around Black
Holes
|
astro-ph
|
For the past twenty-five years, nearly all analyses of accretion disk
dynamics have assumed that stress inside the disk is locally proportional to
pressure (the "alpha-model") and that this stress goes to zero at the
marginally stable orbit. Recently, it has been demonstrated that MHD turbulence
accounts for the bulk of internal disk stress. In contradiction with the
traditional view, the stress from this MHD turbulence does not diminish near
the marginally stable orbit, and the ratio of magnetic stress to pressure rises
sharply there. Examples of the consequences include: an increase in accretion
efficiency that may also be time- and circumstance-dependent; a decrease in the
rate of black hole spin-up by accretion; and generation of disk luminosity
fluctuations. Preliminary results from numerical simulations lend support to
analytic estimates of these effects.
|
No Label
|
No Label
|
astro-ph/9811264
|
Denise Rocha Goncalves
|
Magnetic Fields on the Dynamics of the ICM
|
astro-ph
|
Could the discrepancies found in the determination of mass in clusters of
galaxies, from gravitational lensing data and from X-rays observations, be
consequence of the standard description of the ICM, in which it is assumed
hydrostatic equilibrium maintained by thermal pressure? In analogy to the
interstellar medium of the Galaxy, it is expected a non-thermal term of
pressure, which contains contributions of magnetic fields. We follow the
evolution of the ICM, considering a term of magnetic pressure, aiming at
answering the question whether or not these discrepancies can be explained via
non-thermal terms of pressure. Our results suggest that the magnetic pressure
could only affect the dynamics of the ICM on scales as small as $\la 1 {\rm
kpc}$. These results are compared to the observations of large and small scale
magnetic fields and we are successful at reproducing the available data.
|
No Label
|
No Label
|
astro-ph/0407400
|
N. W. Evans
|
Simulations of the Population of Centaurs I: The Bulk Statistics
|
astro-ph
|
Large-scale simulations of the Centaur population are carried out. The
evolution of 23328 particles based on the orbits of 32 well-known Centaurs is
followed for up to 3 Myr in the forward and backward direction under the
influence of the 4 massive planets. The objects exhibit a rich variety of
dynamical behaviour with half-lives ranging from 540 kyr (1996 AR20) to 32 Myr
(2000 FZ53). The mean half-life of the entire sample of Centaurs is 2.7 Myr.
The data are analyzed using a classification scheme based on the controlling
planets at perihelion and aphelion, previously given in Horner et al (2003).
Transfer probabilities are computed and show the main dynamical pathways of the
Centaur population. The total number of Centaurs with diameters larger than 1
km is estimated as roughly 44300, assuming an inward flux of one new
short-period comet every 200 yrs. The flux into the Centaur region from the
Edgeworth-Kuiper belt is estimated to be 1 new object every 125 yrs. Finally,
the flux from the Centaur region to Earth-crossing orbits is 1 new
Earth-crosser every 880 yrs
|
No Label
|
No Label
|
astro-ph/0402192
|
Takashi Tsuji
|
Dust in the Photospheric Environment II. Effect on the Near Infrared
Spectra of L and T Dwarfs
|
astro-ph
|
We report an attempt to interpret the spectra of L and T dwarfs with the use
of the Unified Cloudy Model (UCM). For this purpose, we extend the grid of the
UCMs to the cases of log g = 4.5 and 5.5. The dust column density relative to
the gas column density in the observable photosphere is larger at the higher
gravities, and molecular line intensity is generally smaller at the higher
gravities. The overall spectral energy distributions (SEDs) are f_{J} < f_{H} <
f_{K} in middle and late L dwarfs, f_{J} < f_{H} > f_{K} in early T dwarfs (L/T
transition objects), and finally f_{J} > f_{H} > f_{K} in middle and late T
dwarfs, where f_{J}, f_{H}, and f_{K} are the peak fluxes at J, H, and K bands,
respectively, in f_{nu} unit. This tendency is the opposite to what is expected
for the temperature effect, but can be accounted for as the effect of thin dust
clouds formed deep in the photosphere together with the effect of the gaseous
opacities including H_2 (CIA), H_2O, CH_4, and K I. Although the UCMs are
semi-empirical models based on a simple assumption that thin dust clouds form
in the region of T_{cr} < T < T_{cond} (T_{cr} = 1800K is an only empirical
parameter while T_{cond} about 2000K is fixed by the thermodynamical data), the
major observations including the overall SEDs as well as the strengths of the
major spectral features are consistently accounted for throughout L and T
dwarfs. In view of the formidable complexities of the cloud formation, we hope
that our UCM can be of some use as a guide for future modelings of the
ultracool dwarfs as well as for interpretation of observed data of L and T
dwarfs.
|
No Label
|
No Label
|
astro-ph/0002437
|
Andrew Barber
|
Weak gravitational lensing in different cosmologies, using an algorithm
for shear in three dimensions
|
astro-ph
|
We present the results of weak gravitational lensing statistics in four
different cosmological $N$-body simulations. The data has been generated using
an algorithm for the three-dimensional shear, which makes use of a variable
softening facility for the $N$-body particle masses, and enables a physical
interpretation for the large-scale structure to be made. Working in
three-dimensions also allows the correct use of the appropriate angular
diameter distances. Our results are presented on the basis of the filled beam
approximation in view of the variable particle softening scheme in our
algorithm. The importance of the smoothness of matter in the universe for the
weak lensing results is discussed in some detail. The low density cosmology
with a cosmological constant appears to give the broadest distributions for all
the statistics computed for sources at high redshifts. In particular, the range
in magnification values for this cosmology has implications for the
determination of the cosmological parameters from high-redshift Type Ia
Supernov\ae. The possibility of determining the density parameter from the
non-Gaussianity in the probability distribution for the convergence is
discussed.
|
No Label
|
No Label
|
astro-ph/9807083
|
van den Berg, S.
|
The Binary Galaxies NGC 147 And NGC 185
|
astro-ph
|
Contrary to a previously published claim it is found that the spheroidal
galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance
estimates place this pair on the near side of the Andromeda subgroup of the
Local Group. The fact that this system has probably remained stable over a
Hubble time suggests that it does not have a plunging orbit that brings it very
close to M 31. It is noted that the only two Local Group galaxy pairs, in which
the components have comparable masses, also have similar morphological types.
NGC 147 and NGC 185 are both spheroidals, while the LMC and SMC are both
irregulars. This suggests that protogalaxies of similar mass that are spawned
in similar environments evolve into objects having similar morphologies.
|
No Label
|
No Label
|
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.