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 RGB 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 [C/N] in data is smaller than theoretical predictions by [0.1615 ± 0.0760 (obs) ± 0.0108 (sys)] 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 [C/Fe] and [N/Fe] in our models, it modestly increases the metallicity-dependent difference relative to the data. We find a well-defined empirical RGBB locus: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>g</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2.6604</mml:mn> <mml:mo>−</mml:mo> <mml:mn>0.1832</mml:mn> <mml:mo stretchy="false">(</mml:mo> <mml:mi>M</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:mn>0.2824</mml:mn> <mml:mspace width="0.25em"/> <mml:mo stretchy="false">[</mml:mo> <mml:mi>Fe</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> </mml:math> . Our model RGBB loci have mass and composition trends that mirror the data, but we find that the observed RGBB is [0.1509 ± 0.0017 (obs) ± 0.0182 (sys)] dex higher than predicted across the board, similar to prior literature results. We find that envelope undershooting, a proposed solution to reconcile theory with data, increases Li destruction during the FDU at higher metallicities, creating tension with depletion observed in GALAH data. We propose Li in the FDU as a sensitive test of the RGBB and FDU, and discuss other potential solutions.