Constraints on the origins of hypervelocity stars: velocity distribution, mergers and star-formation history

TL;DR

This study investigates the origins of hypervelocity stars in the Milky Way halo, finding that their velocity distribution challenges the binary disruption model by the central black hole and suggests alternative formation scenarios.

Contribution

The paper provides new constraints on hypervelocity star origins, showing that binary disruptions by the Galactic Centre black hole cannot fully explain their velocities and distributions.

Findings

Velocity distribution inconsistent with binary disruptions by the MBH

No observed Galactic Centre counterparts match the HVSs

Stellar collisions may explain G2-like dust clouds

Abstract

In recent years surveys have identified several dozen B stars in the Milky Way halo moving faster than the local escape speed. The origin of most of these hypervelocity stars (HVSs) is still poorly constrained. Here we show that the velocity distribution, and in particular the deficiency in >700 km/s HVSs is inconsistent with binary disruptions by the massive black hole (MBH) in the Galactic Centre. This conclusion holds in the full and empty loss cone regime, and for secular instabilities in eccentric disks. Accounting for multiple close encounters between binaries and the MBH, does not qualitatively change the results. Moreover, there is no observed counterpart population in the Galactic Centre that is consistent with the HVSs. The star-formation history could be tuned explain the HVS velocity distribution, but this tuning would produce a mismatch with the observed HVS flight times. Frequent stellar collisions of the binary components due to interactions with the MBH do not significantly impact the velocity distribution in the Galactic halo. Such collisions, however, can leave observable remnants in the Galactic Centre, and potentially explain the origins of G2-like dust clouds.