Runaway collisions in young star clusters - II. Numerical results

By M. Freitag, M. Gurkan, F. W. A. Rechen-Institut, Heidelberg, Heidelberg · Germany, N. University, Evanston, Usa. 2005

186 citations recorded by the source corpus.

We present a new study of the collisional runaway scenario to form an intermediate-mass black hole (IMBH, M-BH greater than or similar to 100M(circle dot)) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (M-* similar or equal to 30-120M(circle dot)) through mass segregation and gravothermal collapse. Previous work established the conditions for this to happen before the massive stars evolve off the main sequence ( MS). In this and a companion paper, we investigate the next stage by implementing direct collisions between stars. Using a Monte Carlo stellar dynamics code, we follow the core collapse and subsequent collisional phase in more than 100 models with varying cluster mass, size, and initial concentration. Collisions are treated either as ideal, 'sticky-sphere' mergers or using realistic prescriptions derived from 3D hydrodynamics computations. In all cases for which the core collapse happens in less than the MS lifetime of massive stars (similar or equal to 3 Myr), we obtain the growth of a single very massive star (VMS, M-* similar or equal to 400-4000M(circle dot)) through a runaway sequence of mergers. Mass loss from collisions, even for velocity dispersions as high as sigma(v) similar to 1000 km s(-1), does not prevent the runaway. The region of cluster parameter space leading to runaway is even more extended than predicted in previous work because, in clusters with sigma(v) > 300 km s(-1), collisions accelerate (and, in extreme cases, drive) core collapse. Although the VMS grows rapidly to greater than or similar to 1000M(circle dot) in models exhibiting runaway, we cannot predict accurately its final mass. This is because the termination of the runaway process must eventually be determined by a complex interplay between stellar dynamics, hydrodynamics, and the stellar evolution of the VMS. In the vast majority of cases, we find that the time between successive collisions becomes much shorter than the thermal time-scale of the VMS. Therefore, our assumption that all stars return quickly to the MS after a collision must eventually break down for the runaway product, and the stellar evolution of the VMS becomes very uncertain. For the same reason, the final fate of the VMS, including its possible collapse to an IMBH, remains unclear.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/63dbefffedf4164bb6108aa70bee484e02d12600

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Citations in the corpus

A Log-Quadratic Relation for Predicting Supermassive Black Hole Masses from the Host Bulge Sérsic Index (2006, background): “To date, such a collisional process in star clusters has only been invoked to explain the formation of intermediate-mass black holes (IMBHs; Quinlan&Shapiro 1990; Portegies Zwart et al. 1999; Freitag et al. 2006).”
Observational evidence for intermediate-mass black holes (2017, background): “…clusters, as stellar clusters are one of the sites of possible IMBH formation (e.g., Portegies Zwart & McMillan 2002; Portegies Zwart et al. 2004; Freitag et al. 2006; Kızıltan et al. 2017); or in the form of off-nuclear ultraluminous X-ray sources (ULXs) in the halos and spiral arms of large…”
STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE (2008, background): “…are naturally expected to lie in relaxed cores in scenarios where the MBH is formed by run-away mergers in the extreme central density following core collapse (Rees 1984), which occurs on timescales much longer then the relaxation time, TR (e.g. Spitzer 1987; Quinlan 1996; Freitag et al. 2006b,c).”
Formation of Supermassive Black Hole Seeds (2016, background): “Their followed up work including stellar dynamics showed that core collapse occurred on ∼ 3 Myr time scale irrespective of the cluster size, mass, concentration and a VMS reached up to 1000 M⊙ (Freitag et al. 2006).”
FORMATION OF THE FIRST NUCLEAR CLUSTERS AND MASSIVE BLACK HOLES AT HIGH REDSHIFT (2008, background): “…that an “intermediate-mass” BH could in principle form as a result of dynamical interactions in dense stellar systems is a long standing idea (Begelman and Rees 1978; Freitag et al. 2006b,a, Ebisuzaki et al. 2001; Portegies Zwart and McMillan 2002; Miller & Hamilton 2002, Gurkan et al. 2004).”
Modeling Dense Star Clusters in the Milky Way and Beyond with the CMC Cluster Catalog (2019, background): “…conditions are met (i.e., if a cluster is sufficiently dense at early times), stellar collisions may lead to a runaway scenario, resulting in the formation of a very massive star and, ultimately, an IMBH (e.g., Portegies Zwart et al. 2004; Freitag et al. 2006; Giersz et al. 2015; Mapelli 2016).”
Massive black hole binaries from runaway collisions: the impact of metallicity (2016, background): “A number of studies adopt models for stellar evolution and/or approximate recipes for mass loss and SNe (Portegies Zwart et al. 1999, 2004; Gürkan et al. 2004; Freitag et al. 2006a,b; Arca-Sedda 2016).”
Ultraluminous X-ray sources in the Chandra and XMM-Newton era (2011, background): “…on runaway core collapse and mergers of O stars inside a compact, young stellar cluster, leading to the possible formation of an IMBH of a few hundred solar masses via direct collapse of the resulting superstar (Portegies Zwart & McMillan 2002; Portegies Zwart et al. 2004; Freitag et al. 2006).”
NEW LIMITS ON AN INTERMEDIATE-MASS BLACK HOLE IN OMEGA CENTAURI. II. DYNAMICAL MODELS (2009, methodology): “…and performed N -body simulations to show that a VMS might grow to MVMS/Mtot ≈ 0.1%, with Mtot being the total cluster mass. Gurkan et al. (2004) and Freitag et al. (2006) used Monte-Carlo simulations with prescriptions and models for stellar collisions and found that the resulting VMS could even…”
The evolution of runaway stellar collision products (2009, background): “Various groups have now confirmed the formation of such collisional runaways, and conjecture that the final merger product collapses to a black hole of up to about 103 M (Portegies Zwart et al. 1999, 2004; Freitag et al. 2006; Gürkan et al. 2006).”
Nuclear star clusters (2009, background): “…mass) black hole within a dense stellar cluster has been discussed for a long time (Begelman and Rees 1978; Quinlan and Shapiro 1987; Ebisuzaki et al. 2001; Miller and Hamilton 2002; Portegies Zwart and McMillan 2002; Portegies Zwart et al. 2004; Gürkan et al. 2004; Freitag et al. 2006b,a).”
Formation of supermassive black holes (2010, background): “…in principle form as a result of dynamical interactions in dense stellar systems is a long standing idea (Begelman and Rees, 1978; Ebisuzaki et al, 2001; Miller and Hamilton, 2002; Portegies Zwart and McMillan, 2002; Portegies Zwart et al, 2004; Freitag et al, 2006b,a; Gürkan et al, 2004, 2006).”