In terms of the standard picture of galactic evolution and structure, early-type stars belong to disk populations. However, a long-standing anomaly has been the existence of a population of early-type stars with near-solar metallicities and main-sequence surface gravities, but high velocities and/or standing at large distances away from the plane. Different hypotheses have been proposed to explain the origin of the high velocity A-stars: they are misidentified abnormal or evolved stars, they are randomly ejected young disk stars, they were formed from a mixture of Galactic gas and gas accreted during the merger of a small satellite galaxy with the Milky Way. In the aim of providing an insight into the different populations of early-type stars, two samples have been gathered from both Hipparcos and Tycho catalogues: high velocity stars in the solar neighbourhood observed by Hipparcos and high galactic latitude faint blue stars observed by Tycho. Parallaxes and proper motions collected by the satellite together with complementary photometric and/or spectroscopic data allow us to study the controversial evolutionary status of these stars in terms of their luminosity, kinematics and metallicity.
IAU Joint Discussion
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