A new route towards merging massive black holes

Abstract

With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellarmass compact objects has become a realistic prospect. Evolutionary scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution, which is poorly understood and leads to large uncertainties in the predicted merger rates. Here we explore, as an alternative, the scenario of massive overcontact binary (MOB) evolution, which involves two very massive stars in a very tight binary that remain fully mixed as a result of their tidally induced high spin. While many of these systems merge early on, we find many MOBs that swap mass several times, but survive as a close binary until the stars collapse. The simplicity of the MOB scenario allows us to use the efficient public stellar-evolution code MESA to explore it systematically by means of detailed numerical calculations. We find that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that will merge within a Hubble time with mass-ratios close to one, in two mass ranges, about 25 . . . 60 M and > ∼ 130 M , with pairinstability supernovae (PISNe) being produced at intermediate masses. Our models are also able to reproduce counterparts of various stages in the MOB scenario in the local Universe, providing direct support for the scenario. We map the initial binary parameter space that produces BH+BH mergers, determine the expected chirp mass distribution, merger times, and expected Kerr parameters, and predict event rates. We find typically one BH+BH merger event for ∼1000 core-collapse supernovae for Z < ∼ Z /10 . The advanced LIGO (aLIGO) detection rate is more uncertain and depends on the cosmic metallicity evolution. From deriving upper and lower limits from a local and a global approximation for the metallicity distribution of massive stars, we estimate aLIGO detection rates (at the aLIGO design limit) of ∼19−550 yr −1 for BH-BH mergers below the PISN gap and of ∼2.1−370 yr −1 above the PISN gap. Even with conservative assumptions, we find that aLIGO will probably soon detect BH+BH mergers from the MOB scenario. These could be the dominant source for aLIGO detections.

Bow shock nebulae of hot massive stars in a magnetized medium

“…Produced before their zero-age mainsequence phase, e.g. by fragmentation of the accretion disk that surrounds massive protostars , those dwarf stars entirely modify the evolution of massive stars and consequently affect their wind properties (de Mink et al 2007(de Mink et al , 2009Paxton et al 2011;Marchant et al 2016).…”

Implications of the gravitational wave event GW150914

“…Although the common envelope phase is considered essential in the standard picture of evolution from isolated massive binary to black hole binary merger, it has been pointed out that other processes could operate if the stars rotate rapidly, or if they have a strong tidal interaction with a close companion [121,122]. The point is that the core-envelope structure develops because once a fused nucleus (say, helium on the main sequence, or carbon or heavier nuclei later on) is produced, it stays in the core and is not replaced by fresh fuel.…”

“…This would lead to a constant resupply of fuel. [121,122] suggest that this could mean that there would never be a giant phase, which could in turn mean that the stars never come into contact with each other. This might also lead to the formation of extra-massive black holes, because the cores that collapse would be large.…”

The effect of pair-instability mass loss on black-hole mergers

“…Finally, prior to the first BH-BH merger detections, Population I/II very massive stars (> 150 M ⊙ ; (Crowther et al 2010)) were also introduced into predictions of BH-BH merger rates (Belczynski et al 2014;Marchant et al 2016).…”

“…PSN are expected to completely disrupt massive stars with no resulting BH formation (Bond et al 1984;Fryer et al 2001;Chatzopoulos & Wheeler 2012a) and thus suppress formation of BH-BH mergers. While PSN are taken into account in some of the predictions for BH-BH merger formation (e.g., Marchant et al (2016);; Spera et al (2016)), PPSN and associated mass loss have thus far been ignored in studies of BH-BH formation (e.g., Dominik et al (2015); Rodriguez et al (2015); Belczynski et al (2016b); Marchant et al (2016); ; Rodriguez et al (2016);; Belczynski et al (2016a); Eldridge & Stanway (2016)) with the exception of recent work by Woosley (2016). We quantify the effect of PPSN and PSN on BH-BH mergers in our isolated classical binary evolution channel.…”