863 research outputs found
The Optical Gravitational Lensing Experiment. Is Interstellar Extinction Toward the Galactic Center Anomalous?
Photometry of the Galactic bulge, collected during the OGLE-II microlensing
search, indicates high and non-uniform interstellar extinction toward the
observed fields. We use the mean I-band magnitude and V-I color of red clump
stars as a tracer of interstellar extinction toward four small regions of the
Galactic bulge with highly variable reddening. Similar test is performed for
the most reddened region observed in the LMC.
We find that the slope of the location of red clump stars in the
color-magnitude diagrams (CMDs) in the Galactic bulge is significantly smaller
than the slope of the reddening line following the standard extinction law
(R_V=3.1) for approximations of the extinction curve by both Cardelli, Clayton
and Mathis (1989, CCM89) and Fitzpatrick (1999, F99). The differences are much
larger for the CCM89 approximation which, on the other hand, indicates the same
slopes for the control field in the LMC, contrary to the F99 approximation. We
discuss possible systematic effects that could cause the observed discrepancy.
Anomalous extinction toward the Galactic bulge seems to be the most natural
explanation. Our data indicate that, generally, the ratio of the total to
selective absorption, R_VI, is much smaller toward the Galactic bulge than the
value corresponding to the standard extinction curve (R_V=3.1). However, R_VI
varies from one line-of-sight to another.
Our results explain why the red clump and RR Lyr stars in the Baade's window
dereddened with standard value of R_VI are redder compared to those of the
local population.Comment: 16 pages. Accepted for publication in ApJ. Major changes include:
comparison of the OGLE-II photometry with other data, additional comparison
of the observed reddening line with that resulting from approximation of the
standard extinction curve by Fitzpatrick (1999
Merging White Dwarf/Black Hole Binaries and Gamma-Ray Bursts
The merger of compact binaries, especially black holes and neutron stars, is
frequently invoked to explain gamma-ray bursts (GRB's). In this paper, we
present three dimensional hydrodynamical simulations of the relatively
neglected mergers of white dwarfs and black holes. During the merger, the white
dwarf is tidally disrupted and sheared into an accretion disk. Nuclear
reactions are followed and the energy release is negligible. Peak accretion
rates are ~0.05 Msun/s (less for lower mass white dwarfs) lasting for
approximately a minute. Many of the disk parameters can be explained by a
simple analytic model which we derive and compare to our simulations. This
model can be used to predict accretion rates for white dwarf and black hole (or
neutron star) masses which are not simulated in this paper. Although the
mergers studied here create disks with larger radii, and longer accretion times
than those from the merger of double neutron stars, a larger fraction of the
merging star's mass becomes part of the disk. Thus the merger of a white dwarf
and a black hole could produce a long duration GRB. The event rate of these
mergers may be as high as 1/Myr per galaxy.Comment: 17 pages text + 9 figures, minor corrections to text and tables,
added references, accepted by Ap
Large Surveys in Cosmology: The Changing Sociology
Galaxy redshift surveys and Cosmic Microwave Background experiments are
undertaken with larger and larger teams, in a fashion reminiscent of particle
physics experiments and the human genome projects. We discuss the role of young
researchers, the issue of multiple authorship, and ways to communicate
effectively in teams of tens to hundreds of collaborators.Comment: Invited article for "Organizations and Strategies in Astronomy II",
ed. A. Heck, Kluwer Acad. Publ., in press (7 pages, no figures
Neutrino Cooled Disk and Its Stability
We investigate the structure and stability of hypercritical accretion flows
around stellar-mass black holes, taking into account neutrino cooling, lepton
conservation, and firstly a realistic equation of state in order to properly
treat the dissociation of nuclei. We obtain the radial distributions of
physical properties, such as density, temperature and electron fraction, for
various mass accretion rates . We find that,
depending on mass accretion rates, different physics affect considerably the
structure of the disk; most important physics is (1) the photodissociation of
nuclei around for relatively low mass accretion rates
(), (2) efficient neutrino cooling
around for moderately high mass accretion rate (), and (3) neutrino trapping () for
very high mass accretion rate (). We
also investigate the stability of hypercritical accretion flows by drawing the
thermal equilibrium curves, and find that efficient neutrino cooling makes the
accretion flows rather stable against both thermal and viscous modes.Comment: 26 pages, 28 figures, Accepted for publication in Ap
Nucleation of quark matter in neutron stars cores
We consider the general conditions of quark droplets formation in high
density neutron matter. The growth of the quark bubble (assumed to contain a
sufficiently large number of particles) can be described by means of a
Fokker-Planck equation. The dynamics of the nucleation essentially depends on
the physical properties of the medium it takes place. The conditions for quark
bubble formation are analyzed within the frameworks of both dissipative and
non-dissipative (with zero bulk and shear viscosity coefficients) approaches.
The conversion time of the neutron star to a quark star is obtained as a
function of the equation of state of the neutron matter and of the microscopic
parameters of the quark nuclei. As an application of the obtained formalism we
analyze the first order phase transition from neutron matter to quark matter in
rapidly rotating neutron stars cores, triggered by the gravitational energy
released during the spinning down of the neutron star. The endothermic
conversion process, via gravitational energy absorption, could take place, in a
very short time interval, of the order of few tens seconds, in a class of dense
compact objects, with very high magnetic fields, called magnetars.Comment: 31 pages, 2 figures, to appear in Ap
Carbon-poor stellar cores as supernova progenitors
Exploring stellar models which ignite carbon off-center (in the mass range of
about 1.05 - 1.25 Msun, depending on the carbon mass fraction) we find that
they may present an interesting SN I progenitor scenario, since whereas in the
standard scenario runaway always takes place at the same density of about 2 X
10^9 gr/cm^3, in our case, due to the small amount of carbon ignited, we get a
whole range of densities from 1 X 10^9 up to 6 X 10^9 gr/cm^3. These results
could contribute in resolving the emerging recognition that at least some
diversity among SNe I exists, since runaway at various central densities is
expected to yield various outcomes in terms of the velocities and composition
of the ejecta, which should be modeled and compared to observations.Comment: 49 pages, 20 figure
Time variability of accretion flows: effects of the adiabatic index and gas temperature
We report on next phase of our study of rotating accretion flows onto black
holes. We consider hydrodynamical (HD) accretion flows with a spherically
symmetric density distribution at the outer boundary but with spherical
symmetry broken by the introduction of a small, latitude-dependent angular
momentum. We study accretion flows by means of numerical two-dimensional,
axisymmetric, HD simulations for variety of the adiabatic index, and
the gas temperature at infinity, . Our work is an extension of work
done by Proga & Begelman who consider models for only . Our main
result is that the flow properties such as the topology of the sonic surface
and time behavior strongly depend on but little on . In
particular, for , the mass accretion rate shows large
amplitude, slow time-variability which is a result of mixing between slow and
fast rotating gas. This temporal behavior differs significantly from that in
models with \gamma\simless 5/3 where the accretion rate is relatively
constant and from that in models with \gamma\simgreat 1 where the accretion
exhibits small amplitude quasi-periodic oscillations. The key parameter
responsible for the differences is the sound speed of the accretion flow which
in turn determines whether the flow is dominated by gas pressure, radiation
pressure or rotation. Despite these differences the time-averaged mass
accretion rate in units of the corresponding Bondi rate is a weak function of
and .Comment: 31 pages, 14 figures, accepted for publication in ApJ, for full
resolution version goto http://users.camk.edu.pl/mmosc/ms.pd
Viscous Torque and Dissipation in the Inner Region of a Thin Accretion Disk: Implications for Measuring Black Hole Spin
We consider a simple Newtonian model of a steady accretion disk around a
black hole. The model is based on height-integrated hydrodynamic equations,
alpha-viscosity, and a pseudo-Newtonian potential that results in an innermost
stable circular orbit (ISCO) that closely approximates the one predicted by GR.
We find that the hydrodynamic models exhibit increasing deviations from the
standard disk model of Shakura & Sunyaev as disk thickness H/R or the value of
alpha increases. The latter is an analytical model in which the viscous torque
is assumed to vanish at the ISCO. We consider the implications of the results
for attempts to estimate black hole spin by using the standard disk model to
fit continuum spectra of black hole accretion disks. We find that the error in
the spin estimate is quite modest so long as H/R < 0.1 and alpha < 0.2. At
worst the error in the estimated value of the spin parameter is 0.1 for a
non-spinning black hole; the error is much less for a rapidly spinning hole. We
also consider the density and disk thickness contrast between the gas in the
disk and that inside the ISCO. The contrast needs to be large if black hole
spin is to be successfully estimated by fitting the relativistically-broadened
X-ray line profile of fluorescent iron emission from reflection off an
accretion disk. In our hydrodynamic models, the contrast in density and
thickness is low when H/R>0.1, sugesting that the iron line technique may be
most reliable in extemely thin disks. We caution that these results have been
obtained with a viscous hydrodynamic model and need to be confirmed with MHD
simulations of radiatively cooled thin disks.Comment: 32 pages, 10 figures; accepted by Ap
An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?
Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with
blackbody temperatures of 50-100 eV and bolometric luminosities above
erg s, and are possibly intermediate mass black holes (IMBHs) of
or massive white dwarfs that are progenitors of type Ia
supernovae. In this letter we report our optical studies of such a source in
M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a
point-like object, the spectral energy distribution of which reveals a blue
component in addition to the companion of an AGB star. The blue component is
consistent with the power-law as expected from the geometrically-thin accretion
disk around an IMBH accretor, but inconsistent with the power-law as expected
from the X-ray irradiated flared accretion disk around a white dwarf accretor.
This result is strong evidence that M81-ULS1 is an IMBH instead of a white
dwarf.Comment: 12 pages, 1 table, 3 figure
The Convective Urca Process with Implicit Two-Dimensional Hydrodynamics
Consideration of the role of the convective flux in the thermodymics of the
convective Urca neutrino loss process in degenerate, convective, quasi-static,
carbon-burning cores shows that the convective Urca process slows down the
convective current around the Urca-shell, but, unlike the "thermal" Urca
process, does not reduce the entropy or temperature for a given convective
volume. Here we demonstrate these effects with two-dimensional numerical
hydrodynamical calculations. These two-dimensional implicit hydrodynamics
calculations invoke an artificial speeding up of the nuclear and weak rates.
They should thus be regarded as indicative, but still qualitative. We find
that, compared to a case with no Urca-active nuclei, the case with Urca effects
leads to a higher entropy in the convective core because the energy released by
nuclear burning is confined to a smaller volume by the effective boundary at
the Urca shell. All else being equal, this will tend to accelerate the
progression to dynamical runaway. We discuss the open issues regarding the
impact of the convective Urca process on the evolution to the "smoldering
phase" and then to dynamical runaway.Comment: 22 pages, 11 figures, accepted for publication in the Astrophysical
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