Forming Young and Hypervelocity Stars in the Galactic Centre via Tidal Disruption of a Molecular Cloud

TL;DR

This paper models how tidal disruption of molecular clouds near the Galactic Centre can produce eccentric stellar disks, hypervelocity stars, and S-stars, matching several observed features and constraining initial cloud parameters.

Contribution

It demonstrates that tidal disruption of molecular clouds can form eccentric stellar disks and hypervelocity stars, providing a unified explanation for multiple phenomena in the Galactic Centre.

Findings

Stars formed have high eccentricities (~0.6) and lopsided configurations.

Secular gravitational instability can produce stars with eccentricities >0.99.

Simulations match the mean eccentricity of the young stellar disk in the Galactic Centre.

Abstract

The Milky Way Galaxy hosts a four million solar mass black hole, Sgr A*, that underwent a major accretion episode approximately 3-6 Myr ago. During the episode, hundreds of young massive stars formed in a disc orbiting Sgr A* in the central half parsec. The recent discovery of a hypervelocity star S5-HVS1, ejected by Sgr A* five Myr ago with a velocity vector consistent with the disc, suggests that this event also produced binary star disruptions. The initial stellar disc has to be rather eccentric for this to occur. Such eccentric disks can form from the tidal disruptions of molecular clouds. Here we perform simulations of such disruptions, focusing on gas clouds on rather radial initial orbits. As a result, stars formed in our simulations are on very eccentric orbits ($\bar{e}\sim 0.6$) with a lopsided configuration. For some clouds counter-rotating stars are formed. As in previous work, we find that such discs undergo a secular gravitational instability that leads to a moderate number of particles obtaining eccentricities of 0.99 or greater, sufficient for stellar binary disruption. We also reproduce the mean eccentricity of the young disk in the Galactic centre, though not the observed surface density profile. We discuss missing physics and observational biases that may explain this discrepancy. We conclude that observed S-stars, hypervelocity stars, and disc stars tightly constrain the initial cloud parameters, indicating a cloud mass between a few$\times 10^4$ and $10^5 M_{\odot}$, and a velocity between $\sim 40$ and 80 km s$^{-1}$ at 10 pc.