Testing stellar yield prescriptions in <scp>omega+</scp> : implications for rising sodium abundances in young thick disc stars

ABSTRACT We recently identified an upturn in [Na/Fe] for the population of solar-type stars in the Galactic young thick disc ($-0.3 &amp;lt; \mathrm{[Fe/H]} &amp;lt; +0.3$ dex) at supersolar metallicity in GALAH (GALactic Archaeology with HERMES) data. Here, we investigate the cause of this unexplained Na enrichment ($[{\rm Na/Fe}] \approx 0 \!-\!0.6$ dex) using the omega+ Galactic chemical evolution code. We investigate the increase of [Na/Fe] with four combinations of nucleosynthetic yields from the literature, with source contributions from core-collapse supernovae (CC SNe), asymptotic giant branch (AGB) stars, and Type Ia supernovae (SNe Ia). We focus on two possible causes for the Na enhancement: the ‘metallicity effect’ resulting from CC SNe at supersolar metallicity and the contribution of metal-rich AGB stars. We adopt two sets of SN Ia yields with one model assuming only Chandrasekhar-mass explosions, and another assuming only sub-Chandrasekhar-mass explosions. We find that the assumed SN Ia explosion has little effect on the [Na/Fe] Galactic chemical evolution modelling, and all Galactic chemical evolution models tested fail to reproduce the observed [Na/Fe] enrichment in the young thick disc population at supersolar metallicities. Our study indicates a possible ‘underpollution effect’ by SNe Ia, which are the dominant producers of iron, in the Galactic disc’s solar-type star population. These findings provide a step forward towards understanding the origin of the unexplained sodium enrichment at supersolar metallicities in the Galactic disc.