The GALAH Survey: Dependence of elemental abundances on age and metallicity for stars in the Galactic disc

Abstract

Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance-age-metallicity relations. Using a stochastic chemical enrichment scheme based on the size of Supernovae remnants, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of main-sequence turn-off stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.

Residual Abundances in GALAH DR3: Implications for Nucleosynthesis and Identification of Unique Stellar Populations

“…These age estimates are derived from PARSEC release v1.2S + COLIBRI stellar isochrones (Marigo et al 2017) and are provided in the GALAH value-added catalog GALAH DR3 VAC ages. We note that our temperature cut of T eff < 6700 excludes many young stars from this diagram, and that stellar ages below or near 1 Gyr may be overestimated (Sharma et al 2020).…”

“…We identify correlations in the residual abundances with age (Sharma et al 2020) for O, K, Y, and Ba. The wings of the K residual abundance distribution increase to ∼ 0.8 dex at young ages, but the core of the distribution remains uncorrelated with age.…”

Presolar grain isotopic ratios as constraints to nuclear and stellar parameters of AGB nucleosynthesis

“…Most (60%) of the grains turned out to be older than the solar system by less than 300 Myr, with a minority showing ages up to 3 Gyr. According to recent estimates of the relations between age and abundances for various representative elements in the galactic disk (Nissen et al 2020;Feuillet et al 2019;Sharma et al 2020) we expect a rather small spread of metallicities across the above mentioned age span at the galactic radius of the solar system. For these reasons, and in agreement with Cristallo et al (2020), our comparisons between model predictions and observed isotopic ratios in SiC grains will be restricted to a small spread, −0.15…”

CoSHA: Code for Stellar properties Heuristic Assignment -- for the MaStar stellar library

“…[α/Fe] vs. [Fe/H] plane is a well characterized plane providing reliable estimates exist for a representative sample of stars, mostly in the MW (e. g. Hayden et al 2015;Sharma et al 2020), the MW neighborhood (e. g., Escala et al 2020;Kirby et al 2020), and integrated stellar populations (e. g., Walcher et al 2016). The importance of this distribution becomes clear since its slope provides links to the past SFH of galaxies.…”