Birth of the Galactic Disk Revealed by the H3 Survey

Understand

We use chemistry ([alpha/Fe] and [Fe/H]), main sequence turnoff ages, and kinematics determined from H3 Survey spectroscopy and Gaia astrometry to identify the birth of the Galactic disk.

• We separate in-situ and accreted stars on the basis of angular momenta and eccentricities.
• The sequence of high-alpha in-situ stars persists down to at least [Fe/H]=-2.5 and shows unexpected non-monotonic behavior: with increasing metallicity the population first declines in [alpha/Fe], then increases over the range -1.3<[Fe/H]<-0.7, and then declines again at higher metallicities.
• The number of stars in the in-situ population rapidly increases above [Fe/H]=-1.