Exploring Sources of Gravitational Waves from Star Cluster Dynamics

Joshua Fuhrman, Aaron Geller, Carl Rodriguez, Fred Rasio
.

Out of 179 simulated clusters we produced 41 black holes existing in binary black hole (BBH) systems that meet our potential LIGO source requirement of an instantaneous inspiral time less than 100 Myr. All BBH systems of interest have been dynamically produced, with about 1 black hole in 1000 entering into a potential LIGO source. This suggests that the vast majority, and possibly all, BBH mergers in modest-sized star clusters depend on dynamical encounters, assuming our initial binary frequency and orbital parameter distributions. Furthermore, as these clusters generally live through 10s-100s of relaxation times, the production of LIGO sources within these star clusters is independent of the initial positions of objects within that cluster. We can identify certain trends in how BBH mergers may be dynamically produced. First, there appears to be a low-N cutoff for LIGO producing star clusters, where those born with between one and five thousand objects are not expected to produce any BBH mergers. These clusters evaporate faster than potential LIGO sources can be dynamically produced, although BBH systems do exist in these modest clusters with larger inspiral times. Second, we observe more relatively large changes in eccentricity than in the semi-major axis of BBH orbits. Finally (and possibly related), we observe that about 80% of potential LIGO BBH systems are part of a triple system at some point in their evolution.

In the future we hope to explore more deeply the observed trends in the dynamical production of BBH LIGO sources. For example, additional low-N simulations can be run to better identify the position and nature of the low-N cutoff for LIGO source producing star clusters. Also, the observed fluctuations in orbital eccentricity combined with the high frequency of triple system suggests that the Kozai mechanism may be important for producing black hole mergers. We can investigate this possibility by taking select inner BBH systems within triple systems and simulating their evolution more precisely and with better handling of the Kozai mechanism (and relativistic corrections when necessary). Lastly a more in depth analysis of binary and triple evolution would be required to confirm which of our identified potential LIGO sources would actually merge.

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This material is based upon work supported by the National Science Foundation under Grant No. AST-1359462, a Research Experiences for Undergraduates (REU) grant awarded to CIERA at Northwestern Universtiy. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

. Joshua Fuhrman
. Northwestern REU Student 2016
. Carnegie Mellon University
. jfuhrman@andrew.cmu.edu