Strong Encounters with Black Holes In Globular Clusters.

Open Access
Clausen, Drew Reid
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
June 04, 2013
Committee Members:
  • Michael Eracleous, Dissertation Advisor
  • Steinn Sigurdsson, Dissertation Advisor
  • Richard Alan Wade, Committee Member
  • Lee S Finn, Committee Member
  • Yuexing Cindy Li, Committee Member
  • black holes
  • globular clusters
  • neutron stars
  • tidal disruption events
As many as 1000 black holes could be produced when a globular cluster's most massive stars end their lives. Following its formation, a globular cluster's black hole population is transformed by the dynamical processes that occur within dense stellar systems. After decades of theoretical and observational study, the outcomes of this evolution remain uncertain. Outstanding questions include the fraction of clusters that retain black holes, the size of the retained population, and the mass distribution of these black holes. In this thesis, we model encounters between black holes and other cluster members to identify observable indicators of the presence and masses of black holes in globular clusters. In one line of research, we model the formation of black hole-neutron star binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, many of the black hole-neutron star binaries formed by these encounters will undergo gravitational radiation driven mergers. Black holes retained by the cluster after merging with a neutron star can acquire subsequent neutron star companions and undergo several mergers. However, the post-merger recoil is only suppressed below the globular cluster escape velocity for black holes with masses exceeding 30 M⊙. Thus, the merger rate is sensitive to the black hole mass distribution. Models with 35 M⊙ black holes predict Advanced LIGO detection rates in the range 0.04 - 0.7 per year. Systems that do not merge may be observable as black hole-millisecond pulsar binaries. We discuss the distribution of orbital parameters in such binaries and the cluster properties that promote their formation. We find that the upper limit for the number of dynamically formed black hole-millisecond pulsar binaries in the Milky Way globular cluster system is ~10. In a complementary study, we model the emission lines produced in the photoionized debris of tidally disrupted white dwarfs and horizontal branch stars. These emission lines can be used to investigate the intermediate mass black holes that may be present in some globular clusters. We find that bright, broad, asymmetric C IV λ1549 and [O III] λ5007 emission lines can be used to identify white dwarf tidal disruption events. When a horizontal branch star is disrupted, the brightest optical emission lines are [N II] λ6583 and [O III] λ5007. We compare our models with two candidate white dwarf tidal disruption events that have been observed in globular clusters. We find several drawbacks to interpreting either of these sources as a white dwarf tidal disruption event. However, models of a red clump horizontal branch star undergoing mild disruption by a 50 - 100 M⊙ black hole yield an emission line spectrum that is in good agreement with one of these sources.