151 research outputs found
Dynamical evolution of rotating dense stellar systems with embedded black holes
Evolution of self-gravitating rotating dense stellar systems (e.g. globular
clusters, galactic nuclei) with embedded black holes is investigated. The
interaction between the black hole and stellar component in differential
rotating flattened systems is followed. The interplay between velocity
diffusion due to relaxation and black hole star accretion is investigated
together with cluster rotation using 2D+1 Fokker-Planck numerical methods. The
models can reproduce the Bahcall-Wolf solution () inside the zone of influence of the black hole. Gravo-gyro and
gravothermal instabilities conduce the system to a faster evolution leading to
shorter collapse times with respect to the non-rotating systems. Angular
momentum transport and star accretion support the development of central
rotation in relaxation time scales. We explore system dissolution due to
mass-loss in the presence of an external tidal field (e.g. globular clusters in
galaxies).Comment: 16 pages, 23 figures, 6 table
Compact Binaries in Star Clusters II - Escapers and Detection Rates
We use a self-consistent Monte Carlo treatment of stellar dynamics to
investigate black hole binaries that are dynamically ejected from globular
clusters to determine if they will be gravitational wave sources. We find that
many of the ejected binaries have initially short periods and will merge within
a Hubble time due to gravitational wave radiation. Thus they are potential
sources for ground-based gravitational wave detectors. We estimate the yearly
detection rate for current and advanced ground-based detectors and find a
modest enhancement over the rate predicted for binaries produced by pure
stellar evolution in galactic fields. We also find that many of the ejected
binaries will pass through the longer wavelength Laser Interferometer Space
Antenna (LISA) band and may be individually resolvable. We find a low
probability that the Galaxy will contain a binary in the LISA band during its
three-year mission. Some such binaries may, however, be detectable at Mpc
distances implying that there may be resolvable stellar-mass LISA sources
beyond our Galaxy. We conclude that globular clusters have a significant effect
on the detection rate of ground-based detectors and may produce interesting
LISA sources in local group galaxies.Comment: 19 pages, 16 figures, 2 tables, submitted to MNRA
2D Fokker-Planck models of rotating clusters
Globular clusters rotate significantly, and with the increasing amount of
detailed morphologicaland kinematical data obtained in recent years on galactic
globular clusters many interesting features show up. We show how our
theoretical evolutionary models of rotating clusters can be used to obtain
fits, which at least properly model the overall rotation and its implied
kinematics in full 2D detail (dispersions, rotation velocities). Our simplified
equal mass axisymmetric rotatingmodel provides detailed two-dimensional
kinematical and morphological data for star clusters. The degree of rotation is
not dominant in energy, but also non-negligible for the phase space
distribution function, shape and kinematics of clusters. Therefore the models
are well applicable for galactic globular clusters. Since previously published
papers on that matter by us made it difficult to do detailed comparisons with
observations we provide a much more comprehensive and easy-to-use set of data
here, which uses as entries dynamical age and flattening of observed cluster
andthen offers a limited range of applicable models in full detail. The method,
data structure and some exemplary comparison with observations are presented.
Future work will improve modelling anddata base to take a central black hole, a
mass spectrum and stellar evolution into account
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
A pilgrimage to gravity on GPUs
In this short review we present the developments over the last 5 decades that
have led to the use of Graphics Processing Units (GPUs) for astrophysical
simulations. Since the introduction of NVIDIA's Compute Unified Device
Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body
simulations and is so popular these days that almost all papers about high
precision N-body simulations use methods that are accelerated by GPUs. With the
GPU hardware becoming more advanced and being used for more advanced algorithms
like gravitational tree-codes we see a bright future for GPU like hardware in
computational astrophysics.Comment: To appear in: European Physical Journal "Special Topics" : "Computer
Simulations on Graphics Processing Units" . 18 pages, 8 figure
Collisional dynamics around binary black holes in galactic centers
We follow the sinking of two massive black holes in a spherical stellar
system where the black holes become bound under the influence of dynamical
friction. Once bound, the binary hardens by three-body encounters with
surrounding stars. We find that the binary wanders inside the core, providing
an enhanced supply of reaction partners for the hardening. The binary evolves
into a highly eccentric orbit leading to coalescence well beyond a Hubble time.
These are the first results from a hybrid ``self consistent field'' (SCF) and
direct Aarseth N-body integrator (NBODY6), which combines the advantages of the
direct force calculation with the efficiency of the field method. The code is
designed for use on parallel architectures and is therefore applicable to
collisional N-body integrations with extraordinarily large particle numbers (>
10^5). This creates the possibility of simulating the dynamics of both globular
clusters with realistic collisional relaxation and stellar systems surrounding
supermassive black holes in galactic nuclei.Comment: 38 pages, 13 figures, submitted to ApJ, accepted, revised text and
added figure
Particle-Particle, Particle-Scaling function (P3S) algorithm for electrostatic problems in free boundary conditions
An algorithm for fast calculation of the Coulombic forces and energies of
point particles with free boundary conditions is proposed. Its calculation time
scales as N log N for N particles. This novel method has lower crossover point
with the full O(N^2) direct summation than the Fast Multipole Method. The
forces obtained by our algorithm are analytical derivatives of the energy which
guarantees energy conservation during a molecular dynamics simulation. Our
algorithm is very simple. An MPI parallelised version of the code can be
downloaded under the GNU General Public License from the website of our group.Comment: 19 pages, 11 figures, submitted to: Journal of Chemical Physic
Accelerating NBODY6 with Graphics Processing Units
We describe the use of Graphics Processing Units (GPUs) for speeding up the
code NBODY6 which is widely used for direct -body simulations. Over the
years, the nature of the direct force calculation has proved a barrier
for extending the particle number. Following an early introduction of force
polynomials and individual time-steps, the calculation cost was first reduced
by the introduction of a neighbour scheme. After a decade of GRAPE computers
which speeded up the force calculation further, we are now in the era of GPUs
where relatively small hardware systems are highly cost-effective. A
significant gain in efficiency is achieved by employing the GPU to obtain the
so-called regular force which typically involves some 99 percent of the
particles, while the remaining local forces are evaluated on the host. However,
the latter operation is performed up to 20 times more frequently and may still
account for a significant cost. This effort is reduced by parallel SSE/AVX
procedures where each interaction term is calculated using mainly single
precision. We also discuss further strategies connected with coordinate and
velocity prediction required by the integration scheme. This leaves hard
binaries and multiple close encounters which are treated by several
regularization methods. The present nbody6-GPU code is well balanced for
simulations in the particle range for a dual GPU system
attached to a standard PC.Comment: 8 pages, 3 figures, 2 tables, MNRAS accepte
Linking the formation and fate of exo-Kuiper belts within solar system analogues
Abstract Escalating observations of exo-minor planets and their destroyed remnants both passing through the solar system and within white dwarf planetary systems motivate an understanding of the orbital history and fate of exo-Kuiper belts and planetesimal discs. Here we explore how the structure of a 40 â 1000 au annulus of planetesimals orbiting inside of a solar system analogue that is itself initially embedded within a stellar cluster environment varies as the star evolves through all of its stellar phases. We attempt this computationally challenging link in four parts: (1) by performing stellar cluster simulations lasting 100 Myr, (2) by making assumptions about the subsequent quiescent 11 Gyr main-sequence evolution, (3) by performing simulations throughout the giant branch phases of evolution, and (4) by making assumptions about the beltâs evolution during the white dwarf phase. Throughout these stages, we estimate the planetesimalsâ gravitational responses to analogues of the four solar system giant planets, as well as to collisional grinding, Galactic tides, stellar flybys, and stellar radiation. We find that the imprint of stellar cluster dynamics on the architecture of âł 100 km-sized exo-Kuiper belt planetesimals is retained throughout all phases of stellar evolution unless violent gravitational instabilities are triggered either (1) amongst the giant planets, or (2) due to a close (âȘ103 au) stellar flyby. In the absence of these instabilities, these minor planets simply double their semimajor axis while retaining their primordial post-cluster eccentricity and inclination distributions, with implications for the free-floating planetesimal population and metal-polluted white dwarfs
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