376 research outputs found
Monte Carlo Simulations of Star Clusters - IV. Calibration of the Monte Carlo Code and Comparison with Observations for the Open Cluster M67
We outline the steps needed in order to incorporate the evolution of single
and binary stars into a particular Monte Carlo code for the dynamical evolution
of a star cluster. We calibrate the results against N-body simulations, and
present models for the evolution of the old open cluster M67 (which has been
studied thoroughly in the literature with N-body techniques). The calibration
is done by choosing appropriate free code parameters. We describe in particular
the evolution of the binary, white dwarf and blue straggler populations, though
not all channels for blue straggler formation are represented yet in our
simulations. Calibrated Monte Carlo runs show good agreement with results of
N-body simulations not only for global cluster parameters, but also for e.g.
binary fraction, luminosity function and surface brightness. Comparison of
Monte Carlo simulations with observational data for M67 shows that is possible
to get reasonably good agreement between them. Unfortunately, because of the
large statistical fluctuations of the numerical data and uncertainties in the
observational data the inferred conclusions about the cluster initial
conditions are not firm.Comment: 15 pages, 24 figure
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
Compact Binaries in Star Clusters I - Black Hole Binaries Inside Globular Clusters
We study the compact binary population in star clusters, focusing on binaries
containing black holes, using a self-consistent Monte Carlo treatment of
dynamics and full stellar evolution. We find that the black holes experience
strong mass segregation and become centrally concentrated. In the core the
black holes interact strongly with each other and black hole-black hole
binaries are formed very efficiently. The strong interactions, however, also
destroy or eject the black hole-black hole binaries. We find no black
hole-black hole mergers within our simulations but produce many hard escapers
that will merge in the galactic field within a Hubble time. We also find
several highly eccentric black hole-black hole binaries that are potential LISA
sources, suggesting that star clusters are interesting targets for space-based
detectors. We conclude that star clusters must be taken into account when
predicting compact binary population statistics.Comment: 19 pages, 5 Tables, 12 Figures, updated in response to referee
report, accepted for publication in MNRA
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
COCOA Code for Creating Mock Observations of Star Cluster Models
We introduce and present results from the COCOA (Cluster simulatiOn
Comparison with ObservAtions) code that has been developed to create idealized
mock photometric observations using results from numerical simulations of star
cluster evolution. COCOA is able to present the output of realistic numerical
simulations of star clusters carried out using Monte Carlo or \textit{N}-body
codes in a way that is useful for direct comparison with photometric
observations. In this paper, we describe the COCOA code and demonstrate its
different applications by utilizing globular cluster (GC) models simulated with
the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically
observe these different GC models with optical telescopes, perform PSF
photometry and subsequently produce observed colour magnitude diagrams. We also
use COCOA to compare the results from synthetic observations of a cluster model
that has the same age and metallicity as the Galactic GC NGC 2808 with
observations of the same cluster carried out with a 2.2 meter optical
telescope. We find that COCOA can effectively simulate realistic observations
and recover photometric data. COCOA has numerous scientific applications that
maybe be helpful for both theoreticians and observers that work on star
clusters. Plans for further improving and developing the code are also
discussed in this paper.Comment: 18 pages, 12 figures, accepted for publication in MNRAS. Revised
manuscript has a new title, better quality figures and many other
improvements. COCOA can be downloaded from: https://github.com/abs2k12/COCOA
(comments are welcome
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