Co-formation of the thin and thick discs revealed by APOGEE-DR16 and
  Gaia-DR2
  Since thin disc stars are younger than thick disc stars on average, the thin
disc is predicted by some models to start forming after the thick disc had
formed, around 10 Gyr ago. Accordingly, no significant old thin disc population
should exist. Using 6-D coordinates from Gaia-DR2 and age estimates from
Sanders & Das (2018), we select $\sim 24000$ old stars (${\tau > 10}$ Gyr, with
uncertainties $\lesssim 15\%$) within 2 kpc from the Sun (full sample). A
cross-match with APOGEE-DR16 ($\sim 1000$ stars) reveals comparable fractions
of old chemically defined thin/thick disc stars. We show that the full sample
pericenter radius ($r_\mathrm{per}$) distribution has three peaks, one
associated with the stellar halo and the other two having contributions from
the thin/thick discs. Using a high-resolution $N$-body+Smooth Particle
Hydrodynamics simulation, we demonstrate that one peak, at
$r_\mathrm{per}\approx 7.1$ kpc, is produced by stars from both discs which
were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the
Solar Neighbourhood, $\sim 1/2$ ($\sim 1/3$) of the old thin (thick) disc stars
are classified as migrators. Our results suggest that thin/thick discs are
affected differently by radial migration inasmuch as they have different
eccentricity distributions, regardless of vertical scale heights. We interpret
the existence of a significant old thin disc population as evidence for an
early co-formation of thin/thick discs, arguing that clump instabilities in the
early disc offer a compelling explanation for the observed trends.
