Hyper-velocity stars

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We propose to study the newly discovered population of hyper-velocity stars in order to determine their true nature. Hyper-velocity stars move so fast that dynamical ejection from a massive black hole (as found e.g. in the Galactic centre) is their only suggested origin and they are becoming more and more recognized as an important tool to understand the history, nature and environment of massive black holes. Moreover, they travel with such extreme velocities that they are unbound to our Galaxy. Although the first hyper-velocity star was discovered only by 2005, already eight additional stars have been found recently (including 4 discoveries by our team) at distances as large as 20 to 110 kpc. We shall embark on a systematic search for candidate hyper-velocity blue stars to be drawn from the Sloan Digital Sky and Hamburg Surveys. Quantitative spectral analyses of high resolution spectra are required to clarify their evolutionary status (young massive or old low mass star) and their age as well as to measure the radial velocity and distance. Astrometry will be carried out to measure the proper motions. From the space motions the stellar path through the Galactic halo will be reconstructed. Although the stars unbound to the Galaxy are most spectacular, young hyper-velocity stars bound to the Galaxy are important to constrain the ejection mechanism, while such stars of large age are probes of the Galactic potential that can be used to constrain the Galactic escape velocity and the mass of the Galaxy.