Abstract details

The results of an ongoing high-resolution, high signal-to-noise spectroscopic study of nearby FGK stars are presented. The sample is volume-complete and thereby provides an unbiased view on the local stellar populations. Kinematics, chemistry, and ages of the investigated stars support the existence of two discrete stellar populations - a thin disk and a thick disk - separated by a star formation gap of no less than three billion years. The thick disk is characterized as an extremely old (>12 Gyr), single-burst population, with a local normalization of 20% among the long-lived stars, which implies that it could be in fact as massive as the thin disk. As such it represents a substantial source of baryonic dark matter, and, in particular, it challenges the paradigm for the hierarchical formation of the Milky Way, or spiral galaxies, in general.