Hypervelocity Stars: Young and Heavy or Old and Light?

Abstract

The first three hyper-velocity stars (HVS) unbound to the Galaxy were serendipitously discovered in 2005. The only suggested origin of hyper-velocity stars is the Galactic Centre as it hosts a super-massive black hole capable of accelerating stars to such high velocities. Only one HVS, the sdO star US 708, is known to be an old low mass star, while HE 0437$-$5439 is an apparently normal early-type B-star, too short-lived to originate from the Galactic Centre, but could possibly come from the LMC. A systematic survey has led to the discovery of seven new HVS of late B-type (similar to the prototype HVS1), which can either be massive stars 3 M(sun) or horizontal branch stars, sufficiently long-lived to have travelled from the Galactic Centre. We present new spectral analyses of five known HVS as well as of a newly discovered candidate. It is possible that the late B-type HVS are a mix of main sequence and evolved BHB stars. In view of the time scale problem we revisit HE 0437$-$5439 and discuss a possible subluminous nature of this star.