“Severe consequences for the final fates of stars”

Stellar-mass black holes with masses of approximately 3-100 times our Sun are the endpoints of massive stars that do not explode in supernovae but collapse into black holes. The progenitors of black holes that lead to mergers are originally born in binary star systems and experience several episodes of mass exchange between the components (Figure 1 and Movie 2): in particular, both black holes are from stars that have been stripped off their envelopes. “The envelope stripping has severe consequences for the final fates of stars. For example, it makes it easier for stars to explode in a supernova and it also leads to universal black hole masses as now predicted by our simulations”, says Philipp Podsiadlowski from Oxford University, second author of the study and currently Klaus Tschira Guest Professor at HITS.
The “stellar graveyard” – a collection of all known masses of the neutron-star and black-hole remains of massive stars – is quickly growing thanks to the ever-increasing sensitivity of the gravitational-wave detectors and ongoing searches for such objects. In particular, there seems to be a gap in the distribution of the chirp masses of merging binary black holes, and evidence emerges for the existence of peaks at roughly 8 and 14 solar masses (Figure 3). These features correspond to the universal chirps predicted by the HITS team. “Any features in the distributions of black-hole and chirp masses can tell us a great deal about how these objects have formed”, says Eva Laplace, the study’s third author.

Not in our galaxy: Black holes with much larger masses

Ever since the first discovery of merging black holes, it became evident that there are black holes with much larger masses than the ones found in our Milky Way. This is a direct consequence of these black holes originating from stars born with a chemical composition different from that in our Milky Way Galaxy. The HITS team could now show that – regardless of the chemical composition – stars that become envelope-stripped in close binaries form black holes of <9 and >16 solar masses but almost none in between.
In merging black holes, the universal black-hole masses of approximately 9 and 16 solar masses logically imply universal chirp masses, i.e. universal sounds. “When updating my lecture on gravitational-wave astronomy, I realized that the gravitational-wave observatories had found first hints of an absence of chirp masses and an overabundance at exactly the universal masses predicted by our models”, says Fabian Schneider. “Because the number of observed black-hole mergers is still rather low, it is not clear yet whether this signal in the data is just a statistical fluke or not”.
Whatever the outcome of future gravitational-wave observations: the results will be exciting and help scientists understand better where the singing black holes in this ocean of voices come from.