622 research outputs found
Evolution of the Binary Fraction in Dense Stellar Systems
Using our recently improved Monte Carlo evolution code, we study the
evolution of the binary fraction in globular clusters. In agreement with
previous N-body simulations, we find generally that the hard binary fraction in
the core tends to increase with time over a range of initial cluster central
densities for initial binary fractions <~ 90%. The dominant processes driving
the evolution of the core binary fraction are mass segregation of binaries into
the cluster core and preferential destruction of binaries there. On a global
scale, these effects and the preferential tidal stripping of single stars tend
to roughly balance, leading to overall cluster binary fractions that are
roughly constant with time. Our findings suggest that the current hard binary
fraction near the half-mass radius is a good indicator of the hard primordial
binary fraction. However, the relationship between the true binary fraction and
the fraction of main-sequence stars in binaries (which is typically what
observers measure) is non-linear and rather complicated. We also consider the
importance of soft binaries, which not only modify the evolution of the binary
fraction, but can drastically change the evolution of the cluster as a whole.
Finally, we describe in some detail the recent addition of single and binary
stellar evolution to our cluster evolution code.Comment: 8 pages, 7 figures in emulateapj format. Submitted to Ap
Binary Stars and Globular Cluster Dynamics
In this brief proceedings article I summarize the review talk I gave at the
IAU 246 meeting in Capri, Italy, glossing over the well-known results from the
literature, but paying particular attention to new, previously unpublished
material. This new material includes a careful comparison of the apparently
contradictory results of two independent methods used to simulate the evolution
of binary populations in dense stellar systems (the direct N-body method of
Hurley, et al. 2007 and the approximate Monte Carlo method of Ivanova, et al.
2005), that shows that the two methods may not actually yield contradictory
results, and suggests future work to more directly compare the two methods.Comment: 7 pages, 1 figure, to appear in "Dynamical Evolution of Dense Stellar
Systems", IAUS 246, ed. E. Vesperin
X-Ray Binaries and the Current Dynamical States of Galactic Globular Clusters
It has been known for over 30 years that Galactic globular clusters (GCs) are
overabundant by orders of magnitude in bright X-ray sources per unit mass
relative to the disk population. Recently a quantitative understanding of this
phenomenon has developed, with a clear correlation between the number of X-ray
sources in a cluster, , and the cluster's encounter frequency, ,
becoming apparent. We derive a refined version of that incorporates
the finite lifetime of X-ray sources and the dynamical evolution of clusters.
With it we find we are able to explain the few clusters that lie off the
-- correlation, and resolve the discrepancy between observed GC
core radii and the values predicted by theory. Our results suggest that most
GCs are still in the process of core contraction and have not yet reached the
thermal equilibrium phase driven by binary scattering interactions.Comment: 4 pages, 1 figure, accepted for publication in ApJ
Monte Carlo Simulations of Globular Cluster Evolution. V. Binary Stellar Evolution
We study the dynamical evolution of globular clusters containing primordial
binaries, including full single and binary stellar evolution using our Monte
Carlo cluster evolution code updated with an adaptation of the single and
binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We
describe the modifications we have made to the code. We present several test
calculations and comparisons with existing studies to illustrate the validity
of the code. We show that our code finds very good agreement with direct N-body
simulations including primordial binaries and stellar evolution. We find
significant differences in the evolution of the global properties of the
simulated clusters using stellar evolution compared to simulations without any
stellar evolution. In particular, we find that the mass loss from stellar
evolution acts as a significant energy production channel simply by reducing
the total gravitational binding energy and can significantly prolong the
initial core contraction phase before reaching the binary-burning quasi steady
state of the cluster evolution as noticed in Paper IV. We simulate a large grid
of clusters varying the initial cluster mass, binary fraction, and
concentration and compare properties of the simulated clusters with those of
the observed Galactic globular clusters (GGCs). We find that our simulated
cluster properties agree well with the observed GGC properties. We explore in
some detail qualitatively different clusters in different phases of their
evolution, and construct synthetic Hertzprung-Russell diagrams for these
clusters.Comment: 46 preprint pages, 18 figures, 3 tables, submitted to Ap
Monte Carlo Simulations of Globular Cluster Evolution. VI. The Influence of an Intermediate Mass Black Hole
We present results of a series of Monte Carlo simulations investigating the
imprint of a central intermediate-mass black hole (IMBH) on the structure of a
globular cluster. We investigate the three-dimensional and projected density
profiles, and stellar disruption rates for idealized as well as realistic
cluster models, taking into account a stellar mass spectrum and stellar
evolution, and allowing for a larger, more realistic, number of stars than was
previously possible with direct N-body methods. We compare our results to other
N-body and Fokker-Planck simulations published previously. We find, in general,
very good agreement for the overall cluster structure and dynamical evolution
between direct N-body simulations and our Monte Carlo simulations. Significant
differences exist in the number of stars that are tidally disrupted by the
IMBH, which is most likely an effect of the wandering motion of the IMBH, not
included in the Monte Carlo scheme. These differences, however, are negligible
for the final IMBH masses in realistic cluster models as the disruption rates
are generally much lower than for single-mass clusters. As a direct comparison
to observations we construct a detailed model for the cluster NGC 5694, which
is known to possess a central surface brightness cusp consistent with the
presence of an IMBH. We find that not only the inner slope but also the outer
part of the surface brightness profile agree well with observations. However,
there is only a slight preference for models harboring an IMBH compared to
models without.Comment: 37 pages, 10 figures, Accepted for publication in ApJ Supplement.
Substantial additions on modeling NGC 5694 since original versio
Effects of stellar collisions on star cluster evolution and core collapse
We systematically study the effects of collisions on the overall dynamical
evolution of dense star clusters using Monte Carlo simulations over many
relaxation times. We derive many observable properties of these clusters,
including their core radii and the radial distribution of collision products.
We also study different aspects of collisions in a cluster taking into account
the shorter lifetimes of more massive stars, which has not been studied in
detail before. Depending on the lifetimes of the significantly more massive
collision products, observable properties of the cluster can be modified
qualitatively; for example, even without binaries, core collapse can sometimes
be avoided simply because of stellar collisions.Comment: 5 pages, 4 figures, Submitted for the IAU symposium, 246 in Capri,
Ital
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