Stellar model tests and age determination for RGB stars from the APO-K2 catalogue

Abstract

By adopting the recently empirically derived dependence of alpha -elements on $ Fe $ instead of the conventionally applied uniform one, we tested the agreement between stellar model predictions and observations for red giant branch (RGB) stars in the APO-K2 catalogue. We particularly focused on the biases in effective temperature scales and on the robustness of age estimations. We computed a grid of stellar models relying on the empirical scaling of alpha -elements, investigating the offset in effective temperature $ T$ between these models and observations, using univariate analyses for both metallicity Fe/H and $ Fe $. To account for potential confounding factors, we then employed a multivariate generalised additive model to study the dependence of $ T$ on Fe/H Fe g$, and stellar mass. The initial analysis revealed a negligible trend of $ T$ with Fe/H in contrast with previous works in the literature, which adopt a uniform relation between the various alpha -elements and $ Fe $. A slight $ T$ difference of 25 K was detected between stars with high and low alpha -enhancement. Our multivariate analysis reveals a dependence of $ T$ on both Fe/H and $ Fe $, and highlights a significant dependence on stellar mass. This suggests a discrepancy in how effective temperature scales with stellar mass in the models compared to observations. Despite differences in assumed chemical composition, our analysis, through a fortunate cancellation effect, yields ages that are largely consistent with recent studies of the same sample. Notably, our analysis identifies a 6<!PCT!> fraction of stars younger than 4 Ga within the high-alpha population. However, our analysis of the C/N ratio supports the possible origin of the these stars as a result of mergers or mass transfer events.