The Prince and The Pauper: Co-evolution of the thin and thick disc in the Milky Way

Matthew Raymond Gent, Phillip Eitner, Chervin F. P. Laporte, Aldo Serenelli, Sergey E. Koposov, Maria Bergemann

Published 2022-06-22Version 1

One of main open questions in the evolution of the Milky Way is the origin of the chemically bi-modal structure of the Galactic disc. Is the bi-modality a manifestation of two populations with a distinct origin? Did these populations co-evolve? Or did the thick disc form before the thin disc, as contemporary chemical evolution models predict. Our goal is to investigate in detail the chemical, temporal, and kinematical structure of the alpha-poor and alpha-rich populations in the Galactic disc. We employ the medium-resolution spectra from the Gaia-ESO large spectroscopic survey, as well as Gaia EDR3 astrometry and photometry. The stellar parameters and chemical abundances are determined using Non-Local Thermodynamic Equilibrium (NLTE) models of synthetic spectra. Ages are computed for a large sample of subgiants using GARSTEC evolutionary tracks. We find a bi-modality in the [$\alpha$/Fe] distributions in the local volume. These distributions are characterised by well defined trends in the space of age- and kinematic (V$_\phi$). We present a significant detection of the metal-poor, down to [Fe/H] $\sim -1$ and [$\alpha$/Fe]-poor component with age of up $\sim$ 10 Gyr. The trend is slightly different for the [$\alpha$/Fe]-rich component, which also spans the entire range of ages from a few to 13 Gyr. However, it is clear that both formed coevally and evolved in parallel.