Strong Encounters With Stellar and Intermediate Mass Black Holes in Globular Clusters
Abstract
It is uncertain whether the black holes (BHs) formed within globular clusters (GCs) are retained. The mass distribution of any remaining BHs is also unknown, but previous studies suggest that GCs could harbor stellar and/or intermediate mass BH (IMBHs). We model encounters between BHs and other GC members to identify observable indicators of the presence and masses of BHs in GCs. We simulate interactions between BH-binaries and single stars to investigate the formation of BH-neutron star (BH-NS) binaries. We find that in dense, massive clusters, many of the BH-NS binaries formed by theses encounters undergo gravitational radiation driven mergers. BHs retained by the cluster after merging with a NS can acquire subsequent NS companions and undergo several mergers. However, the post-merger recoil is only suppressed below the GC escape velocity for BHs with masses exceeding ~30 M⊙. Thus, the merger rate is sensitive to the BH mass distribution. Optimistic estimates of the merger rate imply that this could be the dominant formation channel for BH-NS mergers detectable by AdvLIGO. Systems that do not merge may be observable as BH-millisecond pulsar binaries. We have explored the distribution of orbital parameters in such binaries and the cluster properties that promote their formation. In more extreme encounters, the BH may tidally disrupt a star that passes too closely. Accretion of the disrupted star will produce a luminous flare that can photoionize a portion of the post-disruption debris. Models of the emission line spectrum produced in the debris released when a white dwarf or a horizontal branch star is tidally disrupted by an IMBH suggest that these emission lines can be used to identify such events and to constrain properties of the BH. The predicted optical emission lines are similar to those observed in two extragalactic GCs that host ultraluminous X-ray sources. The interpretation of these sources as tidal disruptions can be tested with Hubble Space Telescope observations that search for the UV emission lines predicted by the models.
- Publication:
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American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22120302C