Massive stars in subparsec rings around galactic centres

S Nayakshin

doi:10.1111/j.1365-2966.2006.10772.x

Abstract

We consider the structure of self-gravitating marginally stable accretion discs in galactic centres in which a small fraction of the disc mass has been converted into protostars. We find that protostars accrete gaseous disc matter at prodigious rates. Mainly due to the stellar accretion luminosity, the disc heats up and thickens geometrically, shutting off further disc fragmentation. The existing protostars, however, continue to gain mass by gas accretion. As a result, the initial mass function for disc-born stars at distances R∼ 0.03–3 pc from the supermassive black hole should be top-heavy. The effect is most pronounced at around R∼ 0.1 pc. We suggest that this result explains observations of rings of young massive stars in our Galaxy and in M31, and we predict that more such rings will be discovered.

Highlights

Accretion discs around supermassive black holes (SMBHs) have been predicted to be gravitationally unstable at large radii where they become too cool to resist self-gravity and can collapse to form stars or planets (Paczynski 1978; Kolykhalov & Sunyaev 1980; Lin & Pringle 1987; Collin & Zahn 1999; Gammie 2001; Goodman 2003)
In a range of distances from the SMBH, interestingly centred at R ∼ 0.1 pc, the creation of the first low-mass protostars should lead to very rapid accretion on these stars
The accretion luminosity of these stars is sufficient to heat the disc up in that range of radii to the point where it becomes stable to self-gravity (Q > 1), which shuts off further fragmentation of the disc

Summary

Introduction

Accretion discs around supermassive black holes (SMBHs) have been predicted to be gravitationally unstable at large radii where they become too cool to resist self-gravity and can collapse to form stars or planets (Paczynski 1978; Kolykhalov & Sunyaev 1980; Lin & Pringle 1987; Collin & Zahn 1999; Gammie 2001; Goodman 2003). In our neighbouring Andromeda galaxy (M31), Bender et al (2005) recently discovered a population of hot blue stars in a disc or ring of similar size, i.e. with a radius of ∼0.15 pc The significance of this discovery is that the SMBH in M31 is determined (e.g. Bender et al 2005) to be as massive as MBH ≈ (1–2) × 108 M , or about 40 times more massive than the SMBH in the Milky Way. The significance of this discovery is that the SMBH in M31 is determined (e.g. Bender et al 2005) to be as massive as MBH ≈ (1–2) × 108 M , or about 40 times more massive than the SMBH in the Milky Way This fact alone rules out (Quataert, private communication) the other plausible mechanism of forming stellar discs around SMBHs, e.g. the massive cluster migration scenario

Results

Discussion

Conclusion