Hypervelocity Stars: Theory driven by discovery and classification

Hypervelocity stars are stars travelling at speeds larger than the escape velocity of the galactic potential. Within the Milky Way, these stars are primarily thought to be produced by binary stars interacting with the super-massive black hole at the centre of the galaxy. Alternative production routes include dynamical interactions of at least 3 stars in regions of high stellar density, binary stars interacting with an intermediate-mass black hole in a cluster, and companion stars being ejected by one component of a binary star going supernova. Rates for each of these routes have been estimated in previous works and predict that the production rate from the super-massive black hole should be dominant.

The first hypervelocity star was discovered by Brown et al. in 2005 and was consistent with a galactic centre origin. Subsequent discoveries, however, have found a significant population of stars with orbits which are inconsistent with a galactic centre origin, including one star with a probable origin in the Large Magellanic Cloud.

The first stage of this work will be to collate a comprehensive catalogue of the current hypervelocity star candidates in the literature. Hypervelocity star candidates span the full range of stellar types and previous studies have only focused on subsets. Kinematic analysis of this catalogue will allow us to test the hypothesis that the dominant production mechanism is the super-massive black hole channel.

The results from the first stage will inform the next, where several alternative mechanisms will be tested against the data collected in the first stage. These mechanisms will be explored analytically and through simulations.

The first two stages are a prelude to the data release from GAIA in the summer of 2016. GAIA will provide precise 6-dimensional kinematics for a sample of at least 10 million stars, allowing the discovery of a large number of hypervelocity stars. We will aim to be among the first people searching through the data for hypervelocity star candidates, and will be well placed to put them in the context of the existing candidates.

The remainder of the PhD will depend on the conclusions on the first year, although preliminary results from our assembled catalogue suggest that there is rich body of work to be done.