# The Correlation Between Mixing Length and Metallicity on the Giant Branch: Implications for Ages in the Gaia Era

## TL;DR

This study reveals a metallicity-dependent temperature offset in red giant stars, suggesting the need for a metallicity-dependent mixing length in stellar models to accurately determine ages in the Gaia era.

## Contribution

It demonstrates that incorporating a metallicity-dependent mixing length improves stellar model agreement with observations, challenging existing convection theories.

## Key findings

- A temperature offset of ~100 K per dex in metallicity is observed.
- Models require a metallicity-dependent mixing length to match data.
- Neglecting this effect can lead to age errors up to a factor of 2.

## Abstract

In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 first ascent red giants. Given the size and accuracy of this sample, these data offer an unprecedented test of the accuracy of stellar models on the post-main-sequence. When we compare these data to theoretical predictions, we find a metallicity dependent temperature offset with a slope of around 100 K per dex in metallicity. We find that this effect is present in all model grids tested and that theoretical uncertainties in the models, correlated spectroscopic errors, and shifts in the asteroseismic mass scale are insufficient to explain this effect. Stellar models can be brought into agreement with the data if a metallicity dependent convective mixing length is used, with $ \Delta\alpha_{\rm ML, YREC} \sim 0.2$ per dex in metallicity, a trend inconsistent with the predictions of three dimensional stellar convection simulations. If this effect is not taken into account, isochrone ages for red giants from the Gaia data will be off by as much as a factor of 2 even at modest deviations from solar metallicity ([Fe/H]=$-$0.5).