The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk

Understand

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation.

• A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr.
• Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars.
• A natural consequence of these two results is that a vertical metallicity gradient is expected in this population.