Estimating stellar birth radii and the time evolution of the Milky Way's ISM metallicity gradient

TL;DR

This paper introduces a semi-empirical method to estimate the birth radii of Milky Way stars and studies the evolution of the galaxy's metallicity gradient over time, revealing insights into disk formation and chemo-kinematics.

Contribution

The authors develop a model-independent approach to determine stellar birth radii using metallicity and age, constraining the ISM metallicity evolution and chemo-kinematical relations without relying on kinematic data.

Findings

The ISM metallicity gradient flattened from -0.15 to -0.07 dex/kpc over time.

Kinematically hot stars likely formed locally or in the outer disk.

The flat local age-metallicity relation results from superimposed mono-r_birth populations.

Abstract

We present a semi-empirical, largely model-independent approach for estimating Galactic birth radii, r_birth, for Milky Way disk stars. The technique relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disk 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_birth distribution. We find that the data are consistent with an ISM radial metallicity gradient that flattens with time from ~-0.15 dex/kpc at the beginning of disk formation, to its measured present-day value (-0.07 dex/kpc). We present several chemo-kinematical relations in terms of mono-r_birth populations. One remarkable result is that the kinematically hottest stars would have been born locally or in the outer disk, consistent with thick disk 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_birth populations, 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.