Galactic archaeology of the Milky Way disc with the Gaia space mission

Abstract

Galactic archaeology aims at unveiling the history of the Milky Way by analysing different stellar properties: chemical abundances, kinematics, dynamics and ages. In particular, the abundance of α-elements (e.g. O, Mg, Si, S, Ca, Ti) with respect to iron ([α/Fe]) is an important fossil signature to trace the chemical evolution of the Galactic disc’s populations, i.e. the thin and the thick disc. The goal of this thesis is to explore the main issues concerning the determination of high-precision abundances from observed spectral data and, by using the high-quality astrometric data provided by the Gaia space mission, analyse the chemodynamical properties of the Galactic disc (e.g. radial abundance gradients, role of stellar migration, age-abundance relations) in order to infer the formation and evolution of the Milky Way.The implemented algorithm was the automated abundance estimation procedure GAUGUIN, which is integrated into the RVS (Radial Velocity Spectrometer) DPAC analysis pipeline of the Gaia mission. We first applied our method to derive [Mg/Fe] abundances for 2210 solar neighbourhood stars observed by the HARPS ESO spectrograph (high spectral resolution: R ∼ 115000), and parametrised by the AMBRE Project (effective temperature, surface gravity, overall metallicity [M/H], [α/Fe] abundance, and radial velocity). We found that the definition of continuum to normalise the observed spectra is responsible for the largest fraction of the uncertainty in the abundance estimate for the metal-rich stellar spectra. We show a methodology that significantly improve the abundance estimate precision, and allows to observe a decreasing trend in the [Mg/Fe] abundance even at supersolar metallicites ([M/H] > 0), partly solving the apparent discrepancies between the observed flat trend in the literature, and the steeper slope predicted by Galactic chemical evolution models.We used these new [Mg/Fe] abundance measurements to interpret the Galactic disc formation and evolution. To this purpose, we estimated ages and orbital properties for 366 main sequence turn-off (MSTO) stars, using PARSEC isochrones, together with astrometric and photometric values from Gaia DR2. We observe a steeper [Mg/Fe] gradient, compared to literature values, which is a direct result of the improvement of the abundance estimates in the metal-rich regime. Moreover, we find a significant spread of stellar ages at any given [Mg/Fe] value, and observe a clear correlated dispersion of the [Mg/Fe] abundance with the metallicity at a given age. While for [M/H] ≤ -0.2, a clear age-metallicity and age-[Mg/Fe] trends are observed, more metal-rich stars display ages from 3 up to 12 Gyr, describing an almost flat trend in the [M/H]-age and [Mg/Fe]-age relations. In addition, we found a significant fraction of radial migrated stars in the Galactic disc, for a wide range of stellar ages. Finally, we observe the appearance of a second chemical sequence at the outer disc, 10-12 Gyr ago. These stars are more metal-poor with respect to the coexisting stellar population in the inner parts of the disc, and show lower [Mg/Fe] abundances than prior disc stars of the same metallicity, leading to a chemical discontinuity. Our data favour the rapid formation of an early disc that settled in the inner regions, followed by the accretion of metal-poor gas, probably related to a major accretion event from a gas-rich satellite merger, that may have triggered the formation of the thin disc population.