Modeling APOKASC-3 red giants: I. The first dredge-up and red giant branch bump

TL;DR

This study compares theoretical models with APOKASC-3 data to analyze key features of red giant stars, revealing discrepancies in chemical abundance changes and the red giant branch bump, and proposing lithium as a diagnostic tool.

Contribution

It provides the first detailed comparison of MESA models with APOKASC-3 data for RGB stars, highlighting systematic differences and proposing lithium as a new diagnostic.

Findings

Observed [C/N] change smaller than models predict

Empirical RGBB locus accurately determined

RGBB brightness higher than theoretical predictions

Abstract

We focus on two key diagnostics of stellar physics in red giant branch (RGB) stars: the first dredge-up (FDU) of nuclear processed material and the location of the red giant branch bump (RGBB). We compare asteroseismic and spectroscopic APOKASC-3 data with theoretical MESA models. Our FDU predictions have similar mass and metallicity trends to the data, but the observed magnitude of the change in $[{\rm C}/{\rm N}]$ in data is smaller than theoretical predictions by $[0.1615 \pm 0.0760 \,({\rm obs}) \pm 0.0108 \,({\rm sys})] \,{\rm dex}$. These results are insensitive to the input physics, but they are at a level consistent with systematic uncertainties in the abundance measurements. When we include observed trends in birth $[{\rm C}/{\rm Fe}]$ and $[{\rm N}/{\rm Fe}]$ in our models, it modestly increases the metallicity dependent difference relative to the data. We find a well-defined empirical RGBB locus: $\log g = 2.6604 - 0.1832 (M/{\rm M}_\odot-1) + 0.2824 \,[{\rm Fe}/{\rm H}]$. Our model RGBB loci have mass and composition trends that mirror the data, but we find that the observed RGBB is $[0.1509 \pm 0.0017 \,({\rm obs}) \pm 0.0182 \,({\rm sys})] \,{\rm dex}$ higher than predicted across the board, similar to prior literature results. We find that envelope undershooting, proposed solution to reconcile theory with data, increases ${\rm Li}$ destruction during the FDU at higher metallicities, creating tension with depletion observed in GALAH data. We propose ${\rm Li}$ in the FDU as a sensitive test of the RGBB and FDU, and discuss other potential solutions.