The AMBRE project: The thick thin disk and thin thick disk of the Milky Way
M. Hayden, A. Recio-Blanco, P. de Laverny, S. Mikolaitis, C. C. Worley

TL;DR
This study uses Gaia and HARPS data to analyze the chemical and orbital properties of Milky Way stars, revealing that the traditional thin/thick disk classification based on chemistry is misleading and should be based on geometry.
Contribution
It provides a detailed analysis of the Milky Way's disk populations, highlighting the importance of geometric definitions over chemical classifications for the thick disk.
Findings
High-[Mg/Fe] stars are coeval with low-[Mg/Fe] stars, indicating simultaneous formation.
Vertical extension of high-[Mg/Fe] stars occurs only among the oldest, most metal-poor stars.
Radial mixing explains the presence of both populations at the solar position.
Abstract
We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from \textit{Gaia}/DR1, providing reliable age estimates with relative uncertainties of Gyr and allowing precise orbital determinations. The sample is split based on chemistry into a low-[Mg/Fe] sequence, which are often identified as thin disk stellar populations, and a high-[Mg/Fe] sequence, which are often associated with the thick disk. We find that the high-[Mg/Fe] chemical sequence has extended star formation for several Gyr and is coeval with the oldest stars of the low-[Mg/Fe] chemical sequence: both the low- and high-[Mg/Fe] sequences were forming stars at the same time. The high-[Mg/Fe] stellar populations are only vertically extended for the oldest, most-metal poor and highest [Mg/Fe] stars. When comparing vertical velocity dispersion…
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