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

TL;DR

This study probes the origin of an observed upturn in $[\mathrm{Na}/\mathrm{Fe}]$ at super-Solar metallicities in Solar-type young thick-disc stars by testing four combinations of nucleosynthetic yields within the OMEGA$+$ Galactic chemical evolution framework. It combines GALAH DR3 data (with $[\mathrm{Na}/\mathrm{Fe}]$ measurements and BSTEP ages) with yield sets for CC SNe, AGB stars (including newly extended high-$Z$ AGB models), and two SN Ia channels (Chandrasekhar and sub-Chandrasekhar). Across a 13 Gyr evolution, none of the models reproduces the observed $[\mathrm{Na}/\mathrm{Fe}]$ upturn at $[\mathrm{Fe/H}]>0$, suggesting that standard yield prescriptions—particularly the SN Ia iron-enrichment pattern—do not fully capture the Na enrichment in metal-rich, young thick-disc stars. The results point toward possible under-pollution by SN Ia iron or missing metallicity-dependent Na production pathways, and highlight the need for yields across metallicities and alternative enrichment channels to reconcile theory with the observed Galactic disc abundances. The work underscores the importance of refining nucleosynthesis yields and mixing processes to interpret elemental trends in the Milky Way accurately.

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.

Figures (6)

• Figure 1: [Na/Fe] as a function of metallicity [Fe/H] for the sample of Solar-type stars from GALAH DR3 used in this work (see Section \ref{['sec:data']}). The left panel (a) shows the distribution coloured by stellar age, illustrating the median stellar age in (Gyr) at each ([Fe/H], [Na/Fe]) bin. The right panel displays the same distribution, highlighting the standard deviation in stellar age within the corresponding bins. The Bayesian Stellar Parameters Estimator (BSTEP) was used to compute the stellar ages. The Solar abundance position is marked by the symbol $(\odot)$ at [Fe/H], [Na/Fe] $= (0,0)$.
• Figure 2: Normalised sodium ($^{23}$Na) production ratios as a function of stellar metallicity (Z) for massive stars with initial masses $15$ and $20$ M$_{\odot}$. Results are shown for models from Woosley1995, Nomoto2013 and Limongi2018. For each model, the $^{23}$Na yield at each metallicity is divided by the minimum yield across all metallicities, highlighting the relative change in Na production with metallicity.
• Figure 3: Spatial coverage of the young thick disc sample in the Solar neighbourhood. Left: Histogram of guiding radii, $\mathrm{R_g}$, for the Solar-type star sample. The vertical dashed line marks the median ($\mathrm{R_g} = 7.96$ kpc); the blue band shows the interquartile range (IQR; $7.36 - 8.52$ kpc). The full extent of the distribution is $5.48 - 11.16$ kpc. Right: Density map of guiding radius versus current Galactocentric radius ($\mathrm{R_g}$ vs. R). The solid diagonal line indicates the one-to-one relation ($\mathrm{R_g} =~$R). The horizontal dashed line and shaded band represent the median and IQR from the left panel. The colour bar indicates the number of stars per bin.
• Figure 4: Bulk galactic properties of our model Galaxy based on adjusted model parameters.
• Figure 5: Panels (a)-(f) are the [Fe/H]-age, age-[Fe/H], [Fe/H]-[Na/Fe], age-[Na/Fe], [Fe/H]-[Na/H] and age-[Na/H] planes, produced by our adjusted GCE code parameters for this study. 'Age' here refers to stellar ages. The grey bins are Solar-type young thick disc stars selected from the GALAH DR3 catalogue using Equation \ref{['eq:Z_range']}, with the yellower area having a higher concentration of stars. Solid lines represent GCE models in which all SN Ia explosions are assumed to be from Chandrasekhar mass WDs, while for the dashed lines, sub-Chandrasekhar mass explosions from WD mergers are assumed. We defined the model label in Table \ref{['tab:model']}. SN Ia contribution is based on the two-exploding white dwarf from Pakmor2022 and delayed detonation for Chandrasekhar-mass white dwarfs, as described in Seitenzahl2013.