Testing stellar yield prescriptions in OMEGA+: Implications for rising sodium abundances in young thick disc stars

TL;DR

This study investigates the unexplained sodium enrichment in young thick disc stars using galactic chemical evolution models, finding that current models cannot fully account for the observed [Na/Fe] increase at super-Solar metallicities.

Contribution

It evaluates different nucleosynthetic yield prescriptions and explosion scenarios, revealing limitations in current models to explain sodium enrichment in the Galactic disc.

Findings

All models fail to reproduce observed Na enrichment.

Type Ia supernova scenario has little impact on Na evolution.

Possible under-pollution effect by Type Ia supernovae suggested.

Abstract

We recently identified an upturn in [Na/Fe] for the population of Solar-type stars in the Galactic young thick disc ($-0.3 < \mathrm{[Fe/H]} < +0.3$) at super-Solar metallicity in data from the GALactic Archaeology with HERMES (GALAH) survey. In this work, we investigate the origin of this unexplained sodium enrichment ([Na/Fe] $\approx 0$--$0.6$~dex) using the OMEGA$+$ galactic chemical evolution code. We explore the rise of [Na/Fe] using four combinations of nucleosynthetic yields from the literature, considering contributions from core-collapse supernovae, asymptotic giant branch stars, and Type~Ia supernovae. Our analysis focuses on two possible drivers of the Na enhancement: a metallicity-dependent increase in Na production from core-collapse supernovae at super-Solar metallicities, and enrichment from metal-rich AGB stars. We adopt two sets of Type~Ia supernova yields, one assuming exclusively Chandrasekhar-mass explosions and the other assuming only sub-Chandrasekhar-mass explosions. We find that the assumed Type~Ia explosion scenario has little influence on the resulting [Na/Fe] evolution, and that all chemical evolution models tested fail to reproduce the observed Na enrichment in the young thick-disc population at super-Solar metallicity. Our results suggest a possible ``under-pollution effect'' by Type~Ia supernovae -- the dominant producers of iron -- in the Solar-type stellar population of the Galactic disc. These findings provide a step toward understanding the origin of the anomalous sodium enrichment at super-Solar metallicities in the Galactic disc.