1,571 research outputs found
The Effect of Stars on the Dark Matter Spike Around a Black Hole: A Tale of Two Treatments
We revisit the role that gravitational scattering off stars plays in
establishing the steady-state distribution of collisionless dark matter (DM)
around a massive black hole (BH). This is a physically interesting problem that
has potentially observable signatures, such as rays from DM
annihilation in a density spike. The system serves as a laboratory for
comparing two different dynamical approaches, both of which have been widely
used: a Fokker-Planck treatment and a two-component conduction fluid treatment.
In our Fokker-Planck analysis we extend a previous analytic model to account
for a nonzero flux of DM particles into the BH, as well as a cut-off in the
distribution function near the BH due to relativistic effects or, further out,
possible DM annihilation. In our two-fluid analysis, following an approximate
analytic treatment, we recast the equations as a "heated Bondi accretion"
problem and solve the equations numerically without approximation. While both
the Fokker-Planck and two-fluid methods yield basically the same DM density and
velocity dispersion profiles away from the boundaries in the spike interior,
there are other differences, especially the determination of the DM accretion
rate. We discuss limitations of the two treatments, including the assumption of
an isotropic velocity dispersion.Comment: 12 pages, 6 figure
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A Representation for Natural Category Systems
Most AI systems model and represent natural concepts and categories using uniform taxonomies,in which no level in the taxonomy is distinguished. W e present a representation of natural taxonomies based on the theory that human category systems are non-uniform.That is, not all levels of abstraction are equally important or useful; there is a basic level which forms the core of a taxonomy. Empirical evidence for this theory is discussed, as are the linguistic and processing implications of this theory for an artificial intelligence/natural language processing system. We present our implementation of this theory in SNePS, a semantic network processing system which includes an A T N parser generator,demonstrating how this design allows our system to model human performance in the natural language generation of the most appropriate category name for an object.The internal structure of categories is also discussed, and a representation for natural concepts using a prototype model is presented and discussed
The Final Fate of Binary Neutron Stars: What Happens After the Merger?
The merger of two neutron stars usually produces a remnant with a mass
significantly above the single (nonrotating) neutron star maximum mass. In some
cases, the remnant will be stabilized against collapse by rapid, differential
rotation. MHD-driven angular momentum transport eventually leads to the
collapse of the remnant's core, resulting in a black hole surrounded by a
massive accretion torus. Here we present simulations of this process. The
plausibility of generating short duration gamma ray bursts through this
scenario is discussed.Comment: 3 pages. To appear in the Proceedings of the Eleventh Marcel
Grossmann Meeting, Berlin, Germany, 23-29 July 2006, World Scientific,
Singapore (2007
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Collapse of Uniformly Rotating Stars to Black Holes and the Formation of Disks
Simulations in general relativity show that the outcome of collapse of a
marginally unstable, uniformly rotating star spinning at the mass-shedding
limit depends critically on the equation of state. For a very stiff equation of
state, which is likely to characterize a neutron star, essentially all of the
mass and angular momentum of the progenitor are swallowed by the Kerr black
hole formed during the collapse, leaving nearly no residual gas to form a disk.
For a soft equation of state with an adiabatic index \Gamma - 4/3 << 1, which
characterizes a very massive or supermassive star supported predominantly by
thermal radiation pressure, as much as 10% of the mass of the progenitor avoids
capture and goes into a disk about the central hole. We present a semi-analytic
calculation that corroborates these numerical findings and shows how the final
outcome of such a collapse may be determined from simple physical
considerations. In particular, we employ a simple energy variational principle
with an approximate, post-Newtonian energy functional to determine the
structure of a uniformly rotating, polytropic star at the onset of collapse as
a function of polytropic index n, where \Gamma = 1+1/n. We then use this data
to calculate the mass and spin of the final black hole and ambient disk. We
show that the fraction of the total mass that remains in the disk falls off
sharply as 3-n (equivalently, \Gamma - 4/3) increases.Comment: 11 pages, 2 figures, 2 tables, AASTeX; accepted to appear in The
Astrophysical Journa
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