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We present a semi-empirical, largely model-independent approach for estimating Galactic birth radii, r birth, for Milky Way disc stars. The technique relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disc lifetime. Stars are projected back to their birth positions according to the observationally derived age and [Fe/H] with no kinematical information required. Applying our approach to the AMBRE:HARPS and HARPS–GTO local samples, we show that we can constrain the ISM metallicity evolution with Galactic radius and cosmic time, [Fe/H]ISM(r, t), by requiring a physically meaningful r birthdistribution. We find that the data are consistent with an ISM radial metallicity gradient that flattens with time from ∼− 0.15 dex kpc−1at the beginning of disc formation, to its measured present-day value (−0.07 dex kpc−1). We present several chemokinematical relations in terms of mono- r birthpopulations. One remarkable result is that the kinematically hottest stars would have been born locally or in the outer disc, consistent with thick disc formation from the nested flares of mono-age populations and predictions from cosmological simulations. This phenomenon can be also seen in the observed age–velocity dispersion relation, in that its upper boundary is dominated by stars born at larger radii. We also find that the flatness of the local age–metallicity relation (AMR) is the result of the superposition of the AMRs of mono- r birthpopulations, each with a well-defined negative slope. The solar birth radius is estimated to be 7.3 ± 0.6 kpc, for a current Galactocentric radius of 8 kpc. [ABSTRACT FROM AUTHOR]

In the last few decades, many efforts have been made to understand the effect of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of the fundamental parameters of the spiral structure is its angular velocity, or pattern speed Ωp, which determines the location of resonances in the disc and the spirals' radial extent. The most direct method for estimating the pattern speed relies on backward integration techniques, trying to locate the stellar birthplace of open clusters. Here, we propose a new method based on the interaction between the spiral arms and the stars in the disc. Using a sample of around 500 open clusters from the New Catalogue of Optically Visible Open Clusters and Candidates, and a sample of 500 giant stars observed by Apache Point Observatory Galactic Evolution Experiment, we find Ωp = 23.0 ± 0.5 km s-1 kpc-1, for a local standard of rest rotation V0 = 220-km s-1 and solar radius R0 = 8.0 kpc. Exploring a range in V0 and R0 within the acceptable values, 200-240 km s-1 and 7.5-8.5 kpc, respectively, results only in a small change in our estimate of Ωp, that is within the error. Our result is in close agreement with a number of studies which suggest values in the range 20-25 km s-1 kpc-1. An advantage of our method is that we do not need knowledge of the stellar age, unlike in the case of the birthplace method, which allows us to use data from large Galactic surveys. The precision of our method will be improved once larger samples of disc stars with spectroscopic information will become available thanks to future surveys such as 4MOST. [ABSTRACT FROM AUTHOR]

In the first paper of this series, we present a new approach for studying the chemo-dynamical evolution in disk galaxies, which consists of fusing disk chemical evolution models with compatible numerical simulations of galactic disks. This method avoids known star formation and chemical enrichment problems encountered in simulations. Here we focus on the Milky Way, by using a detailed thindisk chemical evolution model (matching local observables, which are weakly affected by radial migration) and a simulation in the cosmological context, with dynamical properties close to those of our Galaxy. We examine in detail the interplay between in situ chemical enrichment and radial migration and their impact on key observables in the solar neighborhood, e.g., the age-metallicityvelocity relation, the metallicity distribution, and gradients in the radial and vertical directions. We show that, due to radial migration from mergers at high redshift and the central bar at later times, a sizable fraction of old metal-poor high-[α/Fe] stars can reach the solar vicinity. This naturally accounts for a number of recent observations related to both the thin and thick disks, despite the fact that we use thin-disk chemistry only. Although significant radial mixing is present, the slope in the age-metallicity relation is only weakly affected, with a scatter compatible with recent observational work. While we find a smooth density distribution in the [O/Fe]-[Fe/H] plane, we can recover the observed discontinuity by selecting particles according to kinematic criteria used in high-resolution samples to define the thin and thick disks. We outline a new method for estimating the birth place of the Sun and predict that the most likely radius lies in the range 4.4 < r < 7.7 kpc (for a current location at r = 8 kpc). A new, unifying model for the Milky Way thick disk is offered, where both mergers and radial migration play a role at different stages of the disk evolution. We show that in the absence of early-on massive mergers the vertical velocity dispersion of the oldest stars is underestimated by a factor of ∼2 compared with observations. We can, therefore, argue that the Milky Way thick disk is unlikely to have been formed through a quiescent disk evolution. An observational test involving both chemical and kinematic information must be devised to ascertain this possibility. [ABSTRACT FROM AUTHOR]

Many are the proposed scenarios for the formation of the thick disk of the Milky Way, going from the merger with smaller galaxies to fast gas accretion in the early Universe. Crucial information on which was the dominant mechanism for the formation of the thick disk and other Galactic components is encoded in the chemical properties of their stars. By comparing the chemical properties of bulge, thick and thin disk stars one can address the question of which, among the competing processes of dissipation, satellite accretion and radial migration, play the dominant role in the formation of the different Galactic components. The main difficulty at present is that current data samples suffer from selection biases. Gaia shall bring a quantum leap to the solution of this problem, by providing fiducial samples of the different galactic components with both, chemical and full kinematic information. To pave the road for Gaia, theoretical models need to be improved by taking into account both chemistry and dynamical aspects. A work of this kind is underway in Geneva Observatory. [ABSTRACT FROM AUTHOR]