A new route for the formation of black hole binaries and a prediction for their mass distribution

hs-2007-04-a-full-jpgIn a new Oxford University press release astronomers have revealed an evolutionary route to the formation of stellar-mass black hole binaries such as the one recently seen by Advanced LIGO. While the observed mass deficit of three solar masses exactly fitted Einstein’s predictions, the actual masses, around 30 MSun each, are unexpectedly large. However, tidally induced internal mixing often occurs in massive close binary stars, and careful modelling of this process has revealed new insights into exactly how this affects the masses of the final black holes:

One complication to theories around the formation of tight pairs of compact stars stems from the fact that stars are generally thought to expand when they age. This new theory avoids such an expansion by invoking a mechanism that keeps the interior of very close and sufficiently massive stars completely chemically mixed. The reason for this is that tidal forces keep the stars in bound rotation – that is, they continue to show the same side to each other, similar to how the same side of the Moon always faces the Earth. The fast orbital motion involved in this process leads to extremely rapid rotation of the stars, which triggers internal chemical mixing inside the star. Marchant and colleagues show, through a large number of detailed evolutionary calculations, that these stars never expand and that the binaries remain compact up to the collapse phase, when the stars turn into relatively massive black holes.

An online preprint is available and the abstract (which I reproduce in part) yields the bigger picture:

With recent advances in gravitational-wave astronomy, the direct detection of gravitational waves from the merger of two stellar-mass 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 effcient 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 to 60 MSun and > 130 M, with pair- instability supernovae (PISNe) [no remnant at all] being produced at intermediate masses.

Image: Massive stars in NGC 602 (Hubble 25th Anniversary). Updates: News from the Fermi gamma-ray observatories in orbit: LAT finds no counterpart but GBM data contains a transient event on the same date, and the discoverers themselves write on the implications of GW150914 for a large stochastic gravitational wave background from merging black hole binaries. Now there is a further paper dealing with the weak GBM transient which arrived 0.4 sec after the GW event.


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