The individual abundance distributions of disc stars across birth radii in GALAH

Abstract

Individual abundances in the Milky Way disc record stellar birth properties [e.g. age, birth radius (Rbirth)], and capture the diversity of the star-forming environments over time. Assuming an analytical relationship between ([Fe/H], [α/Fe]), and Rbirth, we examine the distributions of individual abundances [X/Fe] of elements C, O, Mg, Si, Ca (α), Al (odd-z), Mn (iron-peak), Y, and Ba (neutron-capture) for stars in the Milky Way. We want to understand how these elements might differentiate environments across the disc. We assign tracks of Rbirth in the [α/Fe] versus [Fe/H] plane as informed by expectations from simulations for ∼59 000 GALAH stars in the solar neighborhood (R ∼ 7−9 kpc) which also have inferred ages. Our formalism for Rbirth shows that older stars (∼10 Gyrs) have an Rbirth distribution with smaller mean values (i.e. R̄birth ∼ 5 ± 0.8 kpc) compared to younger stars (∼6 Gyrs; R̄birth ∼ 10 ± 1.5 kpc), for a given [Fe/H], consistent with inside–out growth. The α-, odd-z, and iron-peak element abundances decrease as a function of Rbirth, whereas the neutron-capture abundances increase. The Rbirth–[Fe/H] gradient we measure is steeper compared to the present-day gradient (−0.066 dex kpc−1 versus −0.058 dex kpc−1), which we also find true for Rbirth–[X/Fe] gradients. These results (i) showcase the feasibility of relating the birth radius of stars to their element abundances, (ii) demonstrate that the Milky Way abundance gradients across Rbirth have evolved to be shallower over time, and (iii) offer an observational comparison to element abundance distributions in hydrodynamical simulations.