Since the detection in 2015 of the first gravitational waves, astrophysicists have a new "messenger" to study the most violent phenomena in the universe, among which is the merger of two black holes in orbit around each other. The giant interferometers from the LIGO-Virgo (Laser Interferometer Gravitational-Wave Observatory) collaboration have repeatedly detected the signatures of such events, although many characteristics of the compact objects involved remain unknown.

Where do binary systems made up of merging stellar black holes come from? Two massive stars are created in the same interstellar cloud and exchange matter with each other throughout their lives. After millions of years have passed, they implode into a supernova, one after the other, forming a pair of black holes. These orbit around each other and slowly approach each other over several billion years, eventually merging and emitting gravitational waves. But on what basis are the "progenitor" stars selected for this scenario?

A brief episode in the life of the stellar pair seems to be essential: the "common-envelope phase", in which the pair is entirely immersed in a gas. An impressive transfer of mass then occurs between the two stars and the orbit of the binary system rapidly decreases, potentially leading to their merger.

To find out more, the researchers adapted a tool to this particular framework that can simulate stellar hydrodynamic evolution and the interactions between stars. This was then used to describe the evolution of stars potentially ending in a merger.

Their calculations indicate merger rates between 0.2 and 5 per year and per gigaparsec³ (1 Gpc = 3.26 million light-years) in the local universe, i.e. between 1.2 and 3.3 possible annual detections of black hole mergers below 10 solar masses. This figure is compatible with the events detected by LIGO-Virgo during the first observation campaigns.

This result reinforces the method used to study stellar black holes that are likely to merge and manifest themselves via the emission of a gravitational signal. In particular, only black holes that "survive" the common-envelope phase are then able to merge in less time than the lifespan of the universe.