Search

The discovery of a binary comprising a black hole (BH) and a millisecond pulsar (MSP) would yield insights into stellar evolution and facilitate exquisitely sensitive tests of general relativity. Globular clusters (GCs) are known to harbour large MSP populations and recent studies suggest that GCs may also retain a substantial population of stellar mass BHs. We modelled the formation of BH+MSP binaries in GCs through exchange interactions between binary and single stars. We found that in dense, massive clusters most of the dynamically formed BH+MSP binaries will have orbital periods of 2–10 d, regardless of the mass of the BH, the number of BHs retained by the cluster, and the nature of the GC's binary population. The size of the BH+MSP population is sensitive to several uncertain parameters, including the BH mass function, the BH retention fraction, and the binary fraction in GCs. Based on our models, we estimate that there are 0.6 ± 0.2 dynamically formed BH+MSP binaries in the Milky Way GC system, and place an upper limit on the size of this population of ∼10. Interestingly, we find that BH+MSP binaries will be rare even if GCs retain large BH populations. [ABSTRACT FROM AUTHOR]

We model the formation of black hole–neutron star (BH–NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16–61 per cent of the BH–NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with an NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ∼7 M⊙ BH after it merges with an NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH–NS mergers in globular clusters occur within 4 Gyr of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post-merger kick is suppressed and the BH is retained. Models with 35 M⊙ BHs predict Advanced LIGO detection rates in the range 0.04–0.7 yr−1. On the pessimistic end of this range, BH–NS mergers resulting from binary–single star interactions in globular clusters could account for an interesting fraction of all BH–NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH– NS mergers. [ABSTRACT FROM AUTHOR]

ABSTRACT We model the emission lines generated in the photoionized debris of a tidally disrupted horizontal branch (HB) star. We find that at late times, the brightest optical emission lines are [N ii] λλ6548, 6583 and [O iii] λλ4959, 5007. Models of a red clump HB star undergoing mild disruption by a massive (50-100 M⊙) black hole (BH) yield an emission-line spectrum that is in good agreement with that observed in the NGC 1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8−352604. We make predictions for the ultraviolet emission-line spectrum that can verify the tidal disruption scenario and constrain the mass of the BH. [ABSTRACT FROM AUTHOR]

Primordial stars are likely to be very massive (≥ 30 M⊙), form in isolation, and will likely leave black holes as remnants in the centres of their host dark matter haloes. We expect primordial stars to form in haloes in the mass range 106–1010 M⊙. Some of these early black holes, formed at redshifts z≳ 10, could be the seed black holes for a significant fraction of the supermassive black holes (SMBHs) found in galaxies in the local Universe. If the black hole descendants of the primordial stars exist, their merger with nearby SMBHs may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter haloes in a Λ cold dark matter universe. We seed high-redshift dark matter haloes with early black holes, and explore the merger history of the host haloes and the implications of black hole kick velocities arising from their coalescence. The central concentration of low-mass early black holes in present-day galaxies is reduced if they experience even moderate kicks of tens of km s−1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low-mass black holes that were ejected, compared to those still embedded in their parent haloes. Therefore, merger rates with central SMBHs in the largest halo may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N-body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal-free stars with SMBHs in present-day galaxies. [ABSTRACT FROM AUTHOR]