551 research outputs found
Accretion flows: the Role of the Outer Boundary Condition
We investigate the influences of the outer boundary conditions(OBCs) on the
structure of an optically thin accretion flow. We find that OBC plays an
important role in determining the topological structure and the profiles of the
surface density and temperature of the solution, therefore it should be
regarded as a new parameter in the accretion disk model.Comment: 9 pages, 2 figures, to appear in ApJ Letters, Vol. 521, L5
Constraints on the Redshift and Luminosity Distributions of Gamma Ray Bursts in an Einstein-de Sitter Universe
Two models of the gamma ray burst population, one with a standard candle
luminosity and one with a power law luminosity distribution, are chi^2-fitted
to the union of two data sets: the differential number versus peak flux
distribution of BATSE's long duration bursts, and the time dilation and energy
shifting versus peak flux information of pulse duration time dilation factors,
interpulse duration time dilation factors, and peak energy shifting factors.
The differential peak flux distribution is corrected for threshold effects at
low peak fluxes and at short burst durations, and the pulse duration time
dilation factors are also corrected for energy stretching and similar effects.
Within an Einstein-de Sitter cosmology, we place strong bounds on the evolution
of the bursts, and these bounds are incompatible with a homogeneous population,
assuming a power law spectrum and no luminosity evolution. Additionally, under
the implied conditions of moderate evolution, the 90% width of the observed
luminosity distribution is shown to be < 10^2, which is less constrained than
others have demonstrated it to be assuming no evolution. Finally, redshift
considerations indicate that if the redshifts of BATSE's faintest bursts are to
be compatible with that which is currently known for galaxies, a standard
candle luminosity is unacceptable, and in the case of the power law luminosity
distribution, a mean luminosity < 10^57 ph s^-1 is favored.Comment: Accepted to the Astrophysical Journal, 18 pages, LaTe
The Ratio of Total to Selective Extinction Toward Baade's Window
We measure the ratio of total to selective extinction, R_{VI}=A_V/E(V-I),
toward Baade's Window by comparing the VIK colors of 132 Baade's Window G and K
giants from Tiede, Frogel, & Terndrup with the solar-neighborhood (V-I),(V-K)
relation from Bessell & Brett. We find R_{VI}=2.283 +/- 0.016, and show that
our measurement has no significant dependence on stellar type from G0 to K4.
Adjusting the Paczynski et al. determination of the centroid of the dereddened
Baade's Window clump for this revised value of , we find I_{0,RC}=14.43
and (V-I)_{0,RC}=1.058. This implies a distance to the Baade's Window clump of
d_{BW} = 8.63 +/- 0.16 kpc, where the error bar takes account of statistical
but not systematic uncertainties.Comment: 8 pages, 1 figure, submitted to Ap
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
Journa
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
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
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
The Role of Kinetic Energy Flux in the Convective Urca Process
The previous analysis of the convective Urca neutrino loss process in
degenerate, convective, quasi-static, carbon-burning cores by Barkat and
Wheeler omitted specific consideration of the role of the kinetic energy flux.
The arguments of Barkat and Wheeler that steady-state composition gradients
exist are correct, but chemical equilibrium does not result in net cooling.
Barkat and Wheeler included a "work" term that effectively removed energy from
the total energy budget that could only have come from the kinetic energy,
which must remain positive. Consideration of the kinetic energy in the
thermodynamics of the convective Urca process shows that the convective Urca
neutrinos reduce the rate of increase of entropy that would otherwise be
associated with the input of nuclear energy and slow down the convective
current, but, unlike the "thermal" Urca process do not reduce the entropy or
temperature.Comment: 16 pages, AAS LaTex, in press, Astrophysical Journal, September 20,
Vol 52
On the Energetics of Advection-Dominated Accretion Flows
Using mean field MHD, we discuss the energetics of optically thin, two
temperature, advection-dominated accretion flows (ADAFs). If the magnetic field
is tangled and roughly isotropic, flux freezing is insufficient to maintain the
field in equipartition with the gas. In this case, we expect a fraction of the
energy generated by shear in the flow to be used to build up the magnetic field
strength as the gas flows in; the remaining energy heats the particles. We
argue that strictly equipartition magnetic fields are incompatible with a
priori reasonable levels of particle heating; instead, the plasma in
ADAFs (defined to be the gas pressure divided by magnetic/turbulent pressure)
is likely to be \gsim 5; correspondingly, the viscosity parameter is
likely to be \lsim 0.2Comment: 24 pages, ApJ submitte
Black hole-neutron star mergers: effects of the orientation of the black hole spin
The spin of black holes in black hole-neutron star (BHNS) binaries can have a
strong influence on the merger dynamics and the postmerger state; a wide
variety of spin magnitudes and orientations are expected to occur in nature. In
this paper, we report the first simulations in full general relativity of BHNS
mergers with misaligned black hole spin. We vary the spin magnitude from a/m=0
to a/m=0.9 for aligned cases, and we vary the misalignment angle from 0 to 80
degrees for a/m=0.5. We restrict our study to 3:1 mass ratio systems and use a
simple Gamma-law equation of state. We find that the misalignment angle has a
strong effect on the mass of the postmerger accretion disk, but only for angles
greater than ~ 40 degrees. Although the disk mass varies significantly with
spin magnitude and misalignment angle, we find that all disks have very similar
lifetimes ~ 100ms. Their thermal and rotational profiles are also very similar.
For a misaligned merger, the disk is tilted with respect to the final black
hole's spin axis. This will cause the disk to precess, but on a timescale
longer than the accretion time. In all cases, we find promising setups for
gamma-ray burst production: the disks are hot, thick, and hyperaccreting, and a
baryon-clear region exists above the black hole.Comment: 15 pages, 13 figure
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