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

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 /Fe, 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 - /Fe trend with respect to the simple -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 /Fe, reaching a 0. 1 dex difference at /Fe = 0. 3. Notably, the impact on the evolutionary timescale of such oxygen over-abundance with respect to the commonly adopted uniform -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 /Fe = 0. 3, due to the counterbalancing effects of O enrichment on both the evolutionary timescale and the Z-to-Fe/H relationship.