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

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.

Modelling the chemical evolution of Zr, La, Ce, and Eu in the Galactic discs and bulge

“…Moreover, we note that the observed [Eu/Fe] vs. [Fe/H] in the thin disc flattens at high metallicities (see also Delgado-Mena et al 2017). This could be due to radial migration (Schönrich & Binney 2009;Minchev et al 2013Minchev et al , 2018Spitoni et al 2015) (Grisoni et al 2017).…”

The GALAH survey: temporal chemical enrichment of the galactic disc

“…Both the resulting r birth distribution and ISM metallicity gradients are calibrated on AM-BRE:HARPS (De Pascale et al 2014;Hayden et al 2017) and HARPS-GTO (Delgado Mena et al 2017;Adibekyan et al 2012) samples. In this paper, we adopt the predicted ISM metallicity gradients presented in Minchev et al (2018). Left panels of Figure 7 show the distribution of [Fe/H] bulk at different r birth intervals.…”

“…Stars are born inside out, where older stars are more likely to be born in the inner galaxy. We note that the youngest stars have a r birth distribution peaking slightly outside of the solar neighborhood (top right panel), which is most likely due to the ISM metallicity gradients adopted by Minchev et al (2018) were calibrated on the HARPS samples, not optimised for GALAH. These two results are in good agreement with overall trends presented in Minchev et al (2018).…”

“…We note that the youngest stars have a r birth distribution peaking slightly outside of the solar neighborhood (top right panel), which is most likely due to the ISM metallicity gradients adopted by Minchev et al (2018) were calibrated on the HARPS samples, not optimised for GALAH. These two results are in good agreement with overall trends presented in Minchev et al (2018). Comparing to the [Fe/H] distributions at different observed radii bins (r observed ) presented in Hayden et al (2015), we find that our distributions do not change skewness across r birth .…”

“…Tinsley 1979;Edvardsson et al 1993;Ryan et al 1996;Prochaska et al 2000;Bovy et al 2012;Ting et al 2013). The limitation in these age proxies is the significant amount of scatter in abundances at any given age bin (e.g., Minchev et al 2018;Mackereth et al 2017).…”