Effective temperatures of red giants in the APOKASC catalogue and the mixing length calibration in stellar models

TL;DR

This study reanalyzes red giants in the APOGEE-Kepler catalogue, finding that model discrepancies with observed temperatures depend on boundary conditions and chemical composition, especially for alpha-enhanced stars.

Contribution

It demonstrates that the disagreement in effective temperatures can be resolved with alternative models and highlights the importance of boundary conditions in stellar modeling.

Findings

Disagreement disappears for scaled solar metallicity when using new models.

Discrepancies remain for alpha-enhanced stars, underpredicting temperature changes.

Boundary conditions significantly impact temperature predictions in stellar models.

Abstract

Red giants in the updated APOGEE-Kepler catalogue, with estimates of mass, chemical composition, surface gravity and effective temperature, have recently challenged stellar models computed under the standard assumption of solar calibrated mixing length. In this work, we critically reanalyse this sample of red giants, adopting our own stellar model calculations. Contrary to previous results, we find that the disagreement between the effective temperature scale of red giants and models with solar calibrated mixing length disappears when considering our models and the APOGEE-Kepler stars with scaled solar metal distribution. However, a discrepancy shows up when alpha-enhanced stars are included in the sample. We have found that assuming mass, chemical composition and effective temperature scale of the APOGEE-Kepler catalogue, stellar models generally underpredict the change of temperature of red giants caused by alpha-element enhancements at fixed [Fe/H]. A second important conclusion is that the choice of the outer boundary conditions employed in model calculations is critical. Effective temperature differences (metallicity dependent) between models with solar calibrated mixing length and observations appear for some choices of the boundary conditions, but this is not a general result