We present an investigation of the known sample of runaway stars. The orbits of these stars are traced back to their origin in the Galactic disc. The velocity distribution of these stars is compared to theoretical predictions. We conclude that the ma
jority of stars is well explained by the standard binary ejection mechanism (BEM) and the dynamical ejection mechanism (DEM). However, we find a sample of ten stars which has ejection velocities in excess of those predicted by standard scenarios. We discuss how these can be explained by a variant of the BEM. This mechanism can create runaway stars exceeding the Galactic escape velocity (known as hypervelocity stars). The number of runaway stars in our Galaxy is estimated and compared to the known sample of high mass X-ray binaries, whose formation is linked to the BEM channel.
The contribution of white dwarfs of the different Galactic populations to the stellar content of our Galaxy is only poorly known. Some authors claim a vast population of halo white dwarfs, which would be in accordance with some investigations of the
early phases of Galaxy formation claiming a top-heavy initial-mass-function. Here, I present a model of the population of white dwarfs in the Milky Way based on observations of the local white dwarf sample and a standard model of Galactic structure. This model will be used to estimate the space densities of thin disc, thick disc and halo white dwarfs and their contribution to the baryonic mass budget of the Milky Way. One result of this investigation is that white dwarfs of the halo population contribute a large fraction of the Galactic white dwarf number count, but they are not responsible for the lions share of stellar mass in the Milky Way. Another important result is the substantial contribution of the - often neglected - population of thick disc white dwarfs. Misclassification of thick disc white dwarfs is responsible for overestimates of the halo population in previous investigations.
The Gaia mission will provide an unprecedented 3D view of our galaxy, it will obtain astrometric, photometric and spectrographic data for roughly one billion stars. We are particularly interested in the treasure chest of new data Gaia will produce fo
r hot subdwarf B (sdB) stars. In order for Gaia to classify sdBs and estimate parameters model spectra covering a wide parameter range are needed. Here we describe the construction of an extensive grid, which will be used for this purpose.
