The Galactic thick disk: a stellar population in its own right?

Sammanfattning: The Galactic disk is home of many billion stars, one of which isour Sun. The stellar population of which the Sun is a member residesin the vertically thin spiral structure of the disk. There is a seconddisk population, the so-called thick disk, that has somewhat dierentspatial, kinematic and chemical properties as compared to the thindisk. It may be systematically older than the thin disk (Bernkopf et al.2001), with a star-formation hiatus separating the two. Observationsof thick-disk subgiants allow us to probe the chemical properties ofthese stars. As the subgiant evolutionary phase is short, age-datingthese stars is also possible. Are they in fact systematically older thanthe oldest thin-disk stars? This project will take rst steps towardsanswering this question based on new target selections done on DataRelease 1 of the Gaia mission.