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

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 $[\mathrm{C}/\mathrm{N}]$ in data is smaller than theoretical predictions by $[0.1615\pm 0.0760(\mathrm{obs})\pm 0.0108(\mathrm{sys})]\mathrm{dex}$. 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 $[\mathrm{C}/\mathrm{Fe}]$ and $[\mathrm{N}/\mathrm{Fe}]$ in our models, it modestly increases the metallicity dependent difference relative to the data. We find a well-defined empirical RGBB locus: $\log g=2.6604-0.1832(M/{\mathrm{M}}_{\odot }-1)+0.2824[\mathrm{Fe}/\mathrm{H}]$. Our model RGBB loci have mass and composition trends that mirror the data, but we find that the observed RGBB is $[0.1509\pm 0.0017(\mathrm{obs})\pm 0.0182(\mathrm{sys})]\mathrm{dex}$ higher than predicted across the board, similar to prior literature results. We find that envelope undershooting, proposed solution to reconcile theory with data, increases $\mathrm{Li}$ destruction during the FDU at higher metallicities, creating tension with depletion observed in GALAH data. We propose $\mathrm{Li}$ in the FDU as a sensitive test of the RGBB and FDU, and discuss other potential solutions.

32 pages, 22 figures, matches version accepted by ApJ