Origin of massive high-velocity stars in the halo

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Twelve years after their serendipitous discovery in 2005, hyper-velocity stars (HVSs) are more than ever a hot topic and interest will be boosted in the Gaia era which started right now. These objects are late B-type stars that move with very high velocities through our Galaxy, and most of them are even gravitationally unbound to our Milky Way. Initially, the dynamical interaction with the supermassive black hole in the Galactic centre (GC) was their only suggested origin. In the meantime, however, more and more massive high-velocity stars were found that did not originate in the GC and thus challenged this picture. Those stars are extreme cases of so-called runaway stars, a well-known group of kinematically peculiar stars which can be ejected from their environment by dynamical interactions in star clusters or by a supernova explosion disrupting a binary system. An extragalactic origin from the Large Magellanic Cloud or disrupted dwarf satellite galaxies is also considered possible. Our work during the past couple of years has revealed strong links between runaway stars and HVSs. Consequently, trying to understand the one class of stars requires to understand the other class as well. Therefore, we had extended our research projects to a broad range of high-velocity stars. The goal of this project is to understand the origin and nature of massive runaway and HVSs. This requires a combination of quantitative spectroscopic and astrometric/kinematic analyses. Improved model atmospheres and novel spectral diagnostic tools derived in our team shall be applied to high fidelity optical spectra, partly already at hand. Gaia astrometry will be the backbone of the project. These data can be utilized, for example, to improve Galactic mass models and to trace back stellar trajectories to their places of origin. Furthermore a complete sample of high-velocity stars can be established to test population synthesis models. The places of origin as well as the ejection mechanisms will be identified from chemical tagging combined with the information from Gaia astrometry.