Merging binary black holes formed through chemically homogeneous evolution

A century after Einstein’s theory of general relativity predicted them, the Advanced LIGO project finally proved the existence of gravitational waves from merging black holes – and in so doing opened a new window on to the Universe.

The September 2015 observation of gravitational wave GW150914 – a tiny fluctuation in space-time emanating from a collision of two black holes more than one-and-a-half billion light years from Earth – confirmed that stellar-mass binary black holes (two black holes in close orbit around each other) could merge. Such binaries could arise from several possible formation channels, one of those being chemically homogeneous evolution in short-period stellar binaries.

It so happened that even before LIGO made its historic discovery – revealed in February this year – research carried out by Birmingham astrophysicist Professor Ilya Mandel and Professor Selma de Mink from the University of Amsterdam had concluded that chemically homogeneous evolution represented a viable formation channel for heavy black hole binaries. In a paper published around the time of the historic LIGO announcement, they showed that the advanced gravitational wave detectors could observe hundreds of events each year from this channel, and that GW150914’s source was consistent with having been formed this way.

‘When I say it’s consistent, there are multiple possible ways that the LIGO binaries could have formed; it’s not the only way,’ Ilya is quick to stress. ‘With one event, it’s very difficult to say where it came from and where it was formed. This is why we are looking forward to many more events. Once we have a population, it’s important to know all the possibilities that are out there. Otherwise, if you try to match your population to the models, you’ll get the wrong inference.’

In their paper – which is about modelling massive binaries and is the first attempt to carefully consider this specific formation channel – Ilya and Selma describe a new channel for forming binary black holes that is different from the classical scenarios.

‘Stars normally expand as they evolve – leading to mass transfer in binary systems – but this doesn’t happen if the stars are rapidly rotating and subject to strong tidal forces,’ explains Ilya, a member of Birmingham’s Gravitational Wave Group, which has been heavily involved in LIGO since the beginning of the century. ‘We consider what happens if you have two massive stars that are born very close together; stars that are each 50 to 100 times heavier than the Sun, but with an orbital period of only a couple of days. Having massive stars so close means that rotation and tides can mix the material in the stars. So you don’t end up with a core of helium and outer shell of hydrogen; instead they are well mixed. And because things are so well mixed through the star, the star doesn’t end up shrinking and expanding; the whole star shrinks a bit. That means there’s no mass transfer. So the stars can form black holes in situ. Because they are pretty close, the black holes can merge by just emitting gravitational waves in less than the age of the Universe.’

Such episodes are very difficult to observe because they don’t last long and escaping matter obscures the stars’ inner workings. ‘This means standard observations don’t give us a good handle on what’s going on. LIGO enables us to detect them, and the very first LIGO observation is consistent with coming from this channel. So I’m very much looking forward to answering the question: how can we figure out from future LIGO observations which channel is dominant in the formation of BBHs?’

Ilya and Selma’s paper, ‘Merging binary black holes formed through chemically homogeneous evolution in short-period stellar binaries,’ which was published in the Monthly Notices of the Royal Astronomical Society, is one of more than 100 Ilya has co-written on rate predictions for binary mergers leading to gravitational-wave emission, gravitational-wave data analysis and parameter estimation.

‘These papers are steps towards my big, long-term goal of exploring astrophysics with gravitational waves,’ says Russian-born Ilya, who spent several years studying in the US before coming to Birmingham in 2011.

‘I’m interested in the “rock stars” of the stellar world – stars that burn bright and die young. These stars are 50-100 times more massive than the Sun, and they only live for a few million years. The most exciting moments of their lives make up even less time, which is why it is so hard to catch stars in the act of doing kinky things – such as mass transfer – and that’s why gravitational-wave science is so important. So from my point of view, the real excitement of LIGO is yet to come.’