On the formation and evolution of the first Be star in a black hole binary MWC 656

TL;DR

This paper investigates the formation and evolution of MWC 656, the first known Be star with a black hole companion, suggesting a common envelope phase and supernova are key, and predicts a population of similar binaries impacting gravitational wave detection.

Contribution

It provides a detailed evolutionary scenario for MWC 656, highlighting the potential need to revise standard models of Be star and black hole binary formation.

Findings

MWC 656 likely formed through a common envelope phase and supernova explosion.

Approximately 10-100 similar B BH binaries may exist in the Galactic disk.

Estimated detection rate of coalescing BH-NS systems by LIGO/Virgo is up to 0.2 per year.

Abstract

We find that the formation of MWC 656 (the first Be binary containing a black hole) involves a common envelope phase and a supernova explosion. This result supports the idea that a rapidly rotating Be star can emerge out of a common envelope phase, which is very intriguing because this evolutionary stage is thought to be too fast to lead to significant accretion and spin up of the B star. We predict $\sim 10-100$ of B BH binaries to currently reside in the Galactic disk, among which around $1/3$ contain a Be star, but there is only a small chance to observe a system with parameters resembling MWC 656. If MWC 656 is representative of intrinsic Galactic Be BH binary population, it may indicate that standard evolutionary theory needs to be revised. This would pose another evolutionary problem in understanding BH binaries, with BH X-ray Novae formation issue being the prime example. The future evolution of MWC 656 with a $\sim 5$ M$_{\odot}$ black hole and with a $\sim 13$ M$_{\odot}$ main sequence companion on a $\sim 60$ day orbit may lead to the formation of a coalescing BH-NS system. The estimated Advanced LIGO/Virgo detection rate of such systems is up to $\sim 0.2$ yr$^{-1}$. This empirical estimate is a lower limit as it is obtained with only one particular evolutionary scenario, the MWC 656 binary. This is only a third such estimate available (after Cyg X-1 and Cyg X-3), and it lends additional support to the existence of so far undetected BH--NS binaries.