BYNARY BLACK HOLES OF STELLAR ORIGIN

Image: Two black holes are entwined in a gravitational tango in this artist's conception. Credit: NASA

A binary black hole (BBH) is a system consisting of two black holes in close orbit around each other. Binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture, and binary supermassive black holes believed to be a result of galactic mergers.


The existence of stellar-mass binary black holes (and gravitational waves themselves) were finally confirmed when LIGO detected GW150914 (detected September 2015, announced February 2016), a distinctive gravitational wave signature of two merging stellar-mass black holes of around 30 solar masses each, occurring about 1.3 billion light years away.

In a very rapidly rotating star the material move from the hydrogen-rich envelope into the central burning regions and vice versa. If these processes are efficient the star evolves (quasi) chemically homogeneously.

In a recent paper (Mandel & De Mink 2016) the authors investigate the formation mechanism of binary black holes of stellar origin. They consider massive, tight binaries that evolve nearly chemically homogeneously leading to contraction during the evolution and preventing Roche lobe overflow. This evolutionary scenario predicts the formation of two massive helium stars that may eventually collapse to form two stellar-mass black holes.

Image: Artist's conception of a binary star. Credit: Casey Reed

The authors estimate that these binary black holes typically merge 4-11 Gyr after formation. They perform Monte Carlo simulations of the expected merger rate over cosmic time and obtain a merger rate of about 10 Gpc⁻³ yr⁻¹ at redshift z = 0, peaking at twice this rate at z = 0.5. This values are competitive (in terms of expected rates) with the conventional formation scenarios that involve a common envelope phase during isolated binary evolution or dynamical interaction in a dense cluster.

Unlike the conventional isolated binary evolution channel, short time delays are unlikely for this scenario, implying that mergers at high redshift are not expected.

• Mandel & De Mink 2016, MNRAS - Merging binary black holes formed through chemically homogeneous evolution in short-period stellar binaries (arXiv)
• Binary Black Hole - (astro.cardiff.ac.uk)(Wikipedia)