The GALAH survey: temporal chemical enrichment of the galactic disc

Abstract

We present isochrone ages and initial bulk metallicities ([Fe/H] bulk , by accounting for diffusion) of 163,722 stars from the GALAH Data Release 2, mainly composed of main sequence turn-off stars and subgiants (7000K > T eff > 4000K and logg > 3 dex). The local age-metallicity relationship (AMR) is nearly flat but with significant scatter at all ages; the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appear to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick disks in the solar neighborhood. We also present abundance ratio trends for 16 elements as a function of age, across different [Fe/H] bulk bins. In general, we find the trends in terms of [X/Fe] vs age from our far larger sample to be compatible with studies based on small (∼ 100 stars) samples of solar twins but we now extend it to both sub-and super-solar metallicities. The α-elements show differing behaviour: the hydrostatic α-elements O and Mg show a steady decline with time for all metallicities while the explosive α-elements Si, Ca and Ti are nearly constant during the thin disk epoch (ages < ∼ 12 Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu have the opposite trend, thus favouring a primary production from sources with a short time-delay such as core-collapse supernovae over long-delay events such as neutron star mergers.

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances*

“…The difference in Eu abundance ratios between the low-α and high-α populations could be understood if there is a delay in r-process enrichments; the long star formation timescale of the low-α population might have allowed the delayed r-process enrichments to contribute to the chemical evolution. The delay time of r-process events is, however, still under debate (Hotokezaka et al 2018;Côté et al 2019;Lin et al 2020;Skúladóttir & Salvadori 2020). While neutron star mergers, which are associated with short gamma-ray bursts (GRBs), are promising r-process sites, these studies suggested that there should be more r-process production events in the…”

Stars with Photometrically Young Gaia Luminosities Around the Solar System (SPYGLASS) I: Mapping Young Stellar Structures and their Star Formation Histories

“…While stars are thought to be more metal-rich on average at later formation epochs, the age-metallicity relationship in the solar neighborhood appears to be relatively flat beyond an initial early enrichment phase (e.g., Lin et al 2020;Haywood et al 2019), inferring that a single non-age-dependent metallicity distribution will be sufficient to represent the solar neighborhood. Re-gardless of whether metallicity is truly constant with time, it may not influence our detection of young stars, since the nearly age-independent photometry of mainsequence stars means that the age will weigh minimally on the photometry of these stars, regardless of metallicity.…”

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

“…They showed that a number of elements show abundance trends with age and much of the scatter for a given age can be attributed to metallicity. Lin et al (2020), using data from GALAH DR2, also explored trends with both age and metallicity.…”