Making massive stars in the Galactic Centre via accretion onto low-mass stars within an accretion disc

Abstract

The origin of the population of very massive stars observed within $\sim 0.4$ pc of the supermassive black hole in the Galactic Centre is a mystery. Tidal forces from the black hole would likely inhibit {\it in situ} star formation whilst the youth of the massive stars would seem to exclude formation elsewhere followed by transportation (somehow) into the Galactic centre. Here we consider a third way to produce these massive stars from the lower-mass stars contained in the nuclear stellar cluster which surrounds the supermassive black hole. A passing gas cloud can be tidally shredded by the supermassive black hole forming an accretion disc around the black hole. Stars embedded within this accretion disc will accrete gas from the disc via Bondi-Hoyle accretion, where the accretion rate onto a star, $\dot{M}_\star \propto M_\star^2$. This super-exponential growth of accretion can lead to a steep increase in stellar masses, reaching the required 40-50 M$_\odot$ in some cases. The mass growth rate depends sensitively on the stellar orbital eccentricities and their inclinations. The evolution of the orbital inclinations and/or their eccentricities as stars are trapped by the disc, and their orbits are circularised, will increase the number of massive stars produced. Thus accretion onto low-mass stars can lead to a top heavy stellar mass function in the Galactic Centre and other galactic nuclei. The massive stars produced will pollute the environment via supernova explosions and potentially produce compact binaries whose mergers may be detectable by the LIGO-VIRGO gravitational waves observatories.