Element abundances of galactic RGB stars in the APO-K2 catalogue. Dissimilarity in the scaling with [alpha/Fe]

Abstract

We conducted an investigation on the chemical abundances of 4,316 stars in the red giant branch (RGB) phase from the recently released APO-K2 catalogue. Our aim was to characterize the abundance trends of the single elements with afe , mainly focusing on C, N, and O, which are the most relevant for the estimation of stellar ages. The chemical analysis of the RGB sample involved cross-matching data from the APO-K2 catalogue with individual element abundances from APOGEE DR17. The analysis detected a statistically significant difference in the (C+N+O)/Fe -- afe trend with respect to the simple alpha -enhancement scenario. This difference remained robust across different choices for the reference solar mixture and potential zero-point calibrations of C and N abundances. The primary discrepancy was a steeper increase in O/Fe with afe , reaching a 0.1 dex difference at $ = 0.3$. Notably, the impact on the evolutionary timescale of such oxygen over-abundance with respect to the commonly adopted uniform alpha -enhancement is rather limited. We verified that stellar models computed using an ad hoc O-rich mixture sped up the evolution by only 1 at $ = 0.3$, due to the counterbalancing effects of O enrichment on both the evolutionary timescale and the Fe/H relationship.