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Aims. We conducted an investigation on the chemical abundances of 4316 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 α /Fe, mainly focusing on C, N, and O, which are the most relevant for the estimation of stellar ages. Methods. The chemical analysis of the RGB sample involved cross-matching data from the APO-K2 catalogue with individual element abundances from APOGEE DR17. Results. The analysis detected a statistically significant difference in the (C+N+O)/Fe– α /Fe trend with respect to the simple α -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 α /Fe, reaching a 0.1 dex difference at α /Fe = 0.3. Notably, the impact on the evolutionary timescale of such oxygen over-abundance with respect to the commonly adopted uniform α -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 α /Fe = 0.3, due to the counterbalancing effects of O enrichment on both the evolutionary timescale and the Z -to-Fe/H relationship.
Valle et al. (Tue,) studied this question.