164 research outputs found
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
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
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
Massive Black Holes in Star Clusters. I. Equal-mass clusters
In this paper we report results of collisional N-body simulations of the
dynamical evolution of equal-mass star clusters containing a massive central
black hole. Each cluster is composed of between 5,000 to 180,000 stars together
with a central black hole which contains between 0.2% to 10% of the total
cluster mass.
We find that for large enough black hole masses, the central density follows
a power-law distribution with slope \rho \sim r^{-1.75} inside the radius of
influence of the black hole, in agreement with predictions from earlier Fokker
Planck and Monte Carlo models. The tidal disruption rate of stars is within a
factor of two of that derived in previous studies. It seems impossible to grow
an intermediate-mass black hole (IMBH) from a M \le 100 Msun progenitor in a
globular cluster by the tidal disruption of stars, although M = 10^3 Msun IMBHs
can double their mass within a Hubble time in dense globular clusters. The same
is true for the supermassive black hole at the centre of the Milky Way.
Black holes in star clusters will feed mainly on stars tightly bound to them
and the re-population of these stars causes the clusters to expand, reversing
core-collapse without the need for dynamically active binaries. Close
encounters of stars in the central cusp also lead to an increased mass loss
rate in the form of high-velocity stars escaping from the cluster. A companion
paper will extend these results to the multi-mass case.Comment: 15 pages, 8 figures, ApJ in pres
Lifetimes of tidally limited star clusters with different radii
We study the escape rate, dN/dt, from clusters with different radii in a
tidal field using analytical predictions and direct N-body simulations. We find
that dN/dt depends on the ratio R=r_h/r_j, where r_h is the half-mass radius
and r_j the radius of the zero-velocity surface. For R>0.05, the "tidal
regime", there is almost no dependence of dN/dt on R. To first order this is
because the fraction of escapers per relaxation time, t_rh, scales
approximately as R^1.5, which cancels out the r_h^1.5 term in t_rh. For R<0.05,
the "isolated regime", dN/dt scales as R^-1.5. Clusters that start with their
initial R, Ri, in the tidal regime dissolve completely in this regime and their
t_dis is insensitive to the initial r_h. We predicts that clusters that start
with Ri<0.05 always expand to the tidal regime before final dissolution. Their
t_dis has a shallower dependence on Ri than what would be expected when t_dis
is a constant times t_rh. For realistic values of Ri, the lifetime varies by
less than a factor of 1.5 due to changes in Ri. This implies that the
"survival" diagram for globular clusters should allow for more small clusters
to survive. We note that with our result it is impossible to explain the
universal peaked mass function of globular cluster systems by dynamical
evolution from a power-law initial mass function, since the peak will be at
lower masses in the outer parts of galaxies. Our results finally show that in
the tidal regime t_dis scales as N^0.65/w, with w the angular frequency of the
cluster in the host galaxy. [ABRIDGED
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