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

Jamie Tayar, Garrett Somers, Marc H. Pinsonneault, Dennis Stello, Alexey Mints, Jennifer A. Johnson, O. Zamora, D. A. García-Hernández, Claudia Maraston, Aldo Serenelli, Carlos Allende Prieto, Fabienne A. Bastien, Sarbani Basu, J. C. Bird, R. E. Cohen, Katia Cunha, Yvonne Elsworth, Rafael A. García, Leo Girardi, Saskia Hekker, Jon Holtzman, Daniel Huber, Savita Mathur, Szabolcs Mészáros, B. Mosser, Matthew Shetrone, Victor Silva Aguirre, Keivan Stassun, Guy S. Stringfellow, Gail Zasowski, A. Roman-Lopes

Published 2017-04-04Version 1

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).