622 research outputs found

    Evolution of the Binary Fraction in Dense Stellar Systems

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    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

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    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Italy, glossing over the well-known results from the literature, but paying particular attention to new, previously unpublished material.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

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    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, NXN_X, and the cluster's encounter frequency, Γ\Gamma, becoming apparent. We derive a refined version of Γ\Gamma 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 NXN_X--Γ\Gamma 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

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    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

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    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

    ECTESOL Bulletin Volume 2, Issue 2

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    ECTESOL Bulletin Volume 6, Issue 2

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    ECTESOL Bulletin Volume 3, Issue 1

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    Effects of stellar collisions on star cluster evolution and core collapse

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    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

    ECTESOL Bulletin Volume 2, Issue 3

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