266 research outputs found
Direct N-body Simulations
Special high-accuracy direct force summation N-body algorithms and their
relevance for the simulation of the dynamical evolution of star clusters and
other gravitating N-body systems in astrophysics are presented, explained and
compared with other methods. Other methods means here approximate physical
models based on the Fokker-Planck equation as well as other, approximate
algorithms to compute the gravitational potential in N-body systems. Questions
regarding the parallel implementation of direct ``brute force'' N-body codes
are discussed. The astrophysical application of the models to the theory of
relaxing rotating and non-rotating collisional star clusters is presented,
briefly mentioning the questions of the validity of the Fokker-Planck
approximation, the existence of gravothermal oscillations and of rotation and
primordial binaries.Comment: 32 pages, 13 figures, in press in Riffert, H., Werner K. (eds),
Computational Astrophysics, The Journal of Computational and Applied
Mathematics (JCAM), Elsevier Press, Amsterdam, 199
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
A stochastic Monte Carlo approach to model real star cluster evolution, II. Self-consistent models and primordial binaries
The new approach outlined in Paper I (Spurzem \& Giersz 1996) to follow the
individual formation and evolution of binaries in an evolving, equal point-mass
star cluster is extended for the self-consistent treatment of relaxation and
close three- and four-body encounters for many binaries (typically a few
percent of the initial number of stars in the cluster). The distribution of
single stars is treated as a conducting gas sphere with a standard anisotropic
gaseous model. A Monte Carlo technique is used to model the motion of binaries,
their formation and subsequent hardening by close encounters, and their
relaxation (dynamical friction) with single stars and other binaries. The
results are a further approach towards a realistic model of globular clusters
with primordial binaries without using special hardware. We present, as our
main result, the self-consistent evolution of a cluster consisting of 300.000
equal point-mass stars, plus 30.000 equal mass binaries over several hundred
half-mass relaxation times, well into the phase where most of the binaries have
been dissolved and evacuated from the core. In a self-consistent model it is
the first time that such a realistically large number of binaries is evolving
in a cluster with an even ten times larger number of single stars. Due to the
Monte Carlo treatment of the binaries we can at every moment analyze their
external and internal parameters in the cluster as in an N-body simulation.Comment: LaTeX MNRAS Style 21 pages, 34 figures, submitted to MNRAS Nov. 1999,
for preprint, see
ftp://ftp.ari.uni-heidelberg.de/pub/spurzem/warspaper-98.ps.gz for associated
mpeg-files (20 MB and 13 MB, respectively), see
ftp://ftp.ari.uni-heidelberg.de/pub/spurzem/movie1.mpg and
ftp://ftp.ari.uni-heidelberg.de/pub/spurzem/movie2.mp
A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions
Spherically symmetric equal mass star clusters containing a large amount of
primordial binaries are studied using a hybrid method, consisting of a gas
dynamical model for single stars and a Monte Carlo treatment for relaxation of
binaries and the setup of close resonant and fly-by encounters of single stars
with binaries and binaries with each other (three- and four-body encounters).
What differs from our previous work is that each encounter is being integrated
using a highly accurate direct few-body integrator which uses regularized
variables. Hence we can study the systematic evolution of individual binary
orbital parameters (eccentricity, semi-major axis) and differential and total
cross sections for hardening, dissolution or merging of binaries (minimum
distance) from a sampling of several ten thousands of scattering events as they
occur in real cluster evolution including mass segregation of binaries,
gravothermal collapse and reexpansion, binary burning phase and ultimately
gravothermal oscillations. For the first time we are able to present empirical
cross sections for eccentricity variation of binaries in close three- and
four-body encounters. It is found that a large fraction of three-body and
four-body encounters results in merging. Previous cross sections obtained by
Spitzer and Gao for strong encounters can be reproduced, while for weak
encounters non-standard processes like formation of hierarchical triples occur.Comment: 16 pages, 19 figures, Latex in the MN style, submitted to MNRA
Orbital evolution of the Carina dwarf galaxy and self-consistent star formation history determination
We present a new study of the evolution of the Carina dwarf galaxy that
includes a simultaneous derivation of its orbit and star formation history. The
structure of the galaxy is constrained through orbital parameters derived from
the observed distance, proper motions, radial velocity and star formation
history. The different orbits admitted by the large proper motion errors are
investigated in relation to the tidal force exerted by an external potential
representing the Milky Way (MW). Our analysis is performed with the aid of
fully consistent N-body simulations that are able to follow the dynamics and
the stellar evolution of the dwarf system in order to determine
self-consistently the star formation history of Carina. We find a star
formation history characterized by several bursts, partially matching the
observational expectation. We find also compatible results between dynamical
projected quantities and the observational constraints. The possibility of a
past interaction between Carina and the Magellanic Clouds is also separately
considered and deemed unlikely.Comment: Accepted in A&
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