A new evolutionary scenario for the formation of massive black-hole binaries such as M33 X-7 and IC 10 X-1

TL;DR

This paper proposes a new binary evolution scenario involving rotational mixing to explain the formation of massive black-hole binaries like M33 X-7 without requiring mass transfer episodes.

Contribution

It introduces a novel evolutionary channel where rotational mixing prevents stellar expansion, enabling the formation of massive black holes in close binaries.

Findings

Rotational mixing leads to stars remaining compact during evolution.

The scenario naturally explains the formation of massive black holes in close orbits.

It accounts for systems like M33 X-7 without mass transfer episodes.

Abstract

The formation of close massive black-hole binaries is a challenge for binary evolutionary models, especially the intriguing system M33 X-7 which harbours one of the most massive stellar-mass black holes (16 solar masses) orbiting a 70 solar mass O-star every 3.5 days. In standard binary evolution theory an episode of mass transfer or common envelope is inevitable in a binary with such a small orbital period, which complicates the formation of a black hole with such a high mass. To explain this system, we discuss a new binary evolution channel (De Mink et al. 2009), in which rotational mixing plays an important role. In very massive close binaries, tides force the rotation rate of the stars to be so high that rotationally induced mixing becomes very efficient. Helium produced in the center is mixed throughout the envelope. Instead of expanding during their mainsequence evolution (with the inevitable consequence of mass transfer), these stars stay compact, and avoid filling their Roche lobe. They gradually evolve into massive helium stars. This scenario naturally leads to the formation of very massive black holes in a very close orbit with a less evolved massive companion such as M33 X-7.