4,987 research outputs found
Massive Black Holes in Star Clusters. II. Realistic Cluster Models
We have followed the evolution of multi-mass star clusters containing massive
central black holes through collisional N-body simulations done on GRAPE6. Each
cluster is composed of between 16,384 to 131,072 stars together with a black
hole with an initial mass of M_BH=1000 Msun. We follow the evolution of the
clusters under the combined influence of two-body relaxation, stellar mass-loss
and tidal disruption of stars.
The (3D) mass density profile follows a power-law distribution \rho \sim
r^{-\alpha} with slope \alpha=1.55. This leads to a constant density profile of
bright stars in projection, which makes it highly unlikely that core collapse
clusters contain intermediate-mass black holes (IMBHs). Instead globular
clusters containing IMBHs can be fitted with standard King profiles.
The disruption rate of stars is too small to form an IMBH out of a M_BH
\approx 50 Msun progenitor black hole, unless a cluster starts with a central
density significantly higher than what is seen in globular clusters.
Kinematical studies can reveal 1000 Msun IMBHs in the closest clusters. IMBHs
in globular clusters are only weak X-ray sources since the tidal disruption
rate of stars is low and the star closest to the IMBH is normally another black
hole. For globular clusters, dynamical evolution can push compact stars near
the IMBH to distances small enough that they become detectable through
gravitational radiation. If 10% of all globular clusters contain IMBHs,
extragalactic globular clusters could be one of the major sources for {\it
LISA}. (abridged)Comment: 20 pages, 16 figures, ApJ in pres
Long-Term Evolution of Massive Black Hole Binaries. II. Binary Evolution in Low-Density Galaxies
We use direct-summation N-body integrations to follow the evolution of binary
black holes at the centers of galaxy models with large, constant-density cores.
Particle numbers as large as 400K are considered. The results are compared with
the predictions of loss-cone theory, under the assumption that the supply of
stars to the binary is limited by the rate at which they can be scattered into
the binary's influence sphere by gravitational encounters. The agreement
between theory and simulation is quite good; in particular, we are able to
quantitatively explain the observed dependence of binary hardening rate on N.
We do not verify the recent claim of Chatterjee, Hernquist & Loeb (2003) that
the hardening rate of the binary stabilizes when N exceeds a particular value,
or that Brownian wandering of the binary has a significant effect on its
evolution. When scaled to real galaxies, our results suggest that massive black
hole binaries in gas-poor nuclei would be unlikely to reach gravitational-wave
coalescence in a Hubble time.Comment: 13 pages, 8 figure
QYMSYM: A GPU-Accelerated Hybrid Symplectic Integrator That Permits Close Encounters
We describe a parallel hybrid symplectic integrator for planetary system
integration that runs on a graphics processing unit (GPU). The integrator
identifies close approaches between particles and switches from symplectic to
Hermite algorithms for particles that require higher resolution integrations.
The integrator is approximately as accurate as other hybrid symplectic
integrators but is GPU accelerated.Comment: 17 pages, 2 figure
CW high intensity non-scaling FFAG proton drivers
Accelerators are playing increasingly important roles in basic science,
technology, and medicine including nuclear power, industrial irradiation,
material science, and neutrino production. Proton and light-ion accelerators in
particular have many research, energy and medical applications, providing one
of the most effective treatments for many types of cancer. Ultra high-intensity
and high-energy (GeV) proton drivers are a critical technology for
accelerator-driven sub-critical reactors (ADS) and many HEP programs (Muon
Collider). These high-intensity GeV-range proton drivers are particularly
challenging, encountering duty cycle and space-charge limits in the synchrotron
and machine size concerns in the weaker-focusing cyclotrons; a 10-20 MW proton
driver is not presently considered technically achievable with conventional
re-circulating accelerators. One, as-yet, unexplored re-circulating
accelerator, the Fixed-field Alternating Gradient, or FFAG, is an attractive
alternative to the cyclotron. Its strong focusing optics are expected to
mitigate space charge effects, and a recent innovation in design has coupled
stable tunes with isochronous orbits, making the FFAG capable of
fixed-frequency, CW acceleration, as in the classical cyclotron. This paper
reports on these new advances in FFAG accelerator technology and references
advanced modeling tools for fixed-field accelerators developed for and unique
to the code COSY INFINITY.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
Post-Oligarchic Evolution of Protoplanetary Embryos and the Stability of Planetary Systems
We investigate the orbit-crossing time (T_c) of protoplanet systems both with
and without a gas-disk background. The protoplanets are initially with equal
masses and separation (EMS systems) scaled by their mutual Hill's radii. In a
gas-free environment, we find log (T_c/yr) = A+B \log (k_0/2.3). Through a
simple analytical approach, we demonstrate that the evolution of the velocity
dispersion in an EMS system follows a random walk. The stochastic nature of
random-walk diffusion leads to (i) an increasing average eccentricity ~
t^1/2, where t is the time; (ii) Rayleigh-distributed eccentricities
(P(e,t)=e/\sigma^2 \exp(-e^2/(2\sigma^2)) of the protoplanets; (iii) a
power-law dependence of T_c on planetary separation. As evidence for the
chaotic diffusion, the observed eccentricities of known extra solar planets can
be approximated by a Rayleigh distribution. We evaluate the isolation masses of
the embryos, which determine the probability of gas giant formation, as a
function of the dust and gas surface densities.Comment: 15 pages, 13 figures (2 color ones), accepted for publication in Ap
A rigorous formulation of the cosmological Newtonian limit without averaging
We prove the existence of a large class of one-parameter families of
cosmological solutions to the Einstein-Euler equations that have a Newtonian
limit. This class includes solutions that represent a finite, but otherwise
arbitrary, number of compact fluid bodies. These solutions provide exact
cosmological models that admit Newtonian limits but, are not, either implicitly
or explicitly, averaged
The variable OVIII Warm Absorber in MCG-6-30-15
We present the results of a 4 day ASCA observation of the Seyfert galaxy
MCG-6-30-15, focussing on the nature of the X-ray absorption by the warm
absorber, characterizd by the K-edges of the intermediately ionized oxygen,
OVII and OVIII. We confirm that the column density of OVIII changes on a
timescale of ~s when the X-ray continuum flux decreases. The
significant anti-correlation of column density with continuum flux gives direct
evidence that the warm absorber is photoionized by the X-ray continuum. From
the timescale of the variation of the OVIII column density, we estimate that it
originates from gas within a radius of about 10^{17}\cm of the central
engine. In contrast, the depth of the OVII edge shows no response to the
continuum flux, which indicates that it originates in gas at larger radii. Our
results strongly suggest that there are two warm absorbing regions; one located
near or within the Broad Line Region, the other associated with the outer
molecular torus, scattering medium or Narrow Line Region.Comment: 8 pages (including figures) uuencoded gziped PS file. Submitted to
Publications of the Astronomical Society of Japa
Parameters of core-collapse
This paper considers the phenomenon of deep core collapse in collisional
stellar systems, with stars of equal mass. The collapse takes place on some
multiple, , of the central relaxation time, and produces a density
profile in which , where is a constant. The
parameters and have usually been determined from simplified
models, such as gas and Fokker-Planck models, often with the simplification of
isotropy. Here we determine the parameters directly from N-body simulations
carried out using the newly completed GRAPE-6.Comment: 5 pages, 3 figures, accepted by MNRAS. Reference added to Table
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