On the Rarity of Double Black Hole Binaries: Consequences for Gravitational Wave Detection

Abstract

Double black hole binaries are among the most important sources of gravitational radiation for groundbased detectors such as LIGO or VIRGO. Even if formed with lower efficiency than double neutron star binaries, they could dominate the predicted detection rates, since black holes are more massive than neutron stars and therefore could be detected at greater distances. Here we discuss an evolutionary process that can very significantly limit the formation of close double black hole binaries: the vast majority of their potential progenitors undergo a common envelope (CE) phase while the donor, one of the massive binary components, is evolving through the Hertzsprung gap. Our latest theoretical understanding of the CE process suggests that this will probably lead to a merger, inhibiting double black hole formation. Barring uncertainties in the physics of CE evolution, we use population synthesis calculations, and find that the corresponding reduction in the merger rate of double black holes formed in galactic fields is so great (by ∼ 500) that their contribution to inspiral detection rates for ground-based detectors could become relatively small (∼ 1 in 10) compared to double neutron star binaries. A similar process also reduces the merger rates for double neutron stars, by factor of ∼ 5, eliminating most of the previously predicted ultracompact NS-NS systems. Our predicted detection rates for Advanced LIGO are now much lower for double black holes (∼ 2 yr −1 ), but are still quite high for double neutron stars (∼ 20 yr −1 ). If double black holes were found to be dominant in the detected inspiral signals, this could indicate that they mainly originate from dense star clusters (not included here) or that our theoretical understanding of the CE phase requires significant revision.

The Lowest-Mass Stellar Black Holes: Catastrophic Death of Neutron Stars in Gamma-Ray Bursts

“…3 Northwestern University. 4 Warsaw University.Galactic NS-NS merger rate in the range ∼10-100 Myr Ϫ1 (Belczynski et al 2007), in good agreement with the empirical estimate of ∼3-190 Myr Ϫ1 (Kim et al 2006). The spread in our predicted rates originates from including the most significant model uncertainties associated with the treatment of dynamical mass transfer episodes (common-envelope phases), which are involved in the formation of most double compact objects (Belczynski et al 2007).…”

“…The number of BH-NS binaries that both merge and produce GRBs is hard to estimate since (i) no such system has yet been observed, (ii) formation models are rather uncertain and predict very small BH-NS merger rates (likely too small to explain most of the short bursts), and (iii) theory suggests that the fraction of BH-NS mergers producing bursts depends sensitively on the black hole spin and spin-orbit orientation (Belczynski et al 2008b), but black hole birth spins are not well constrained observationally or theoretically. On the other hand, NS-NS binaries are observed only in the Milky Way, but their properties and numbers are also in agreement with theoretical models, and their merger rate is sufficient to explain the present-day short-burst population (Nakar 2007;Belczynski et al 2007).We have performed an extensive theoretical study of highmass binary stars (potential progenitors of NS-NS systems) using StarTrack, a population synthesis code incorporating the most up-to-date and detailed input physics for massive stars (Belczynski et al 2008a). The code employs state-of-the-art predictions for neutron star and black hole masses based on hydrodynamic core collapse simulations (Fryer & Kalogera 2001) and detailed stellar structure and evolution calculations for massive stars (Timmes et al 1996).…”

Implications of the gravitational wave event GW150914

“…A key part of the evolution of an isolated binary to a coalescing double black hole system is that if the stars are close enough that the giant's envelope overlaps the companion, the companion experiences gas drag and thus spirals inward, reducing the separation. If the star has a smooth density gradient rather than a relatively sharp core-envelope structure, the companion could spiral all the way in and thus reduce BH-BH merger rates for solar metallicity systems [117]. If the companion is a neutron star or black hole, this might produce a "Thorne-Żytkow object" [118,119] in which the core consists of a compact object while the rest of the star looks like a relatively normal giant (although more investigation needs to be performed to determine whether such configurations are stable).…”

Gravitational lensing as a probe of compact object populations in the Galaxy

“…We ignore the contribution of double black hole systems, and single black holes formed in mergers of double black holes as they represent a much smaller population than the ones mentioned above (Belczynski et al 2006).…”

“…In the first model, black holes receive no kicks at all (model K0) and in the second one the kick distribution is the same for black holes as for neutron stars (model K1). Finally in model C we allow for survival in the CE evolution initiated by stars passing through the HG as opposed to the standard model in which we assume that such cases always lead to a merger (Belczynski et al 2007).…”