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

8 pages, 10 figures, Astronomy & Astrophysics, in press

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• Benchmark ages for the Gaia benchmark stars
• Stellar models with calibrated convection and temperature stratification from 3D hydrodynamics simulations
• Coupling 1D stellar evolution with 3D-hydrodynamical simulations on-the-fly II: Stellar Evolution and Asteroseismic Applications
• Cannibals in the thick disk II -- Radial-velocity monitoring of the young alpha-rich stars
• On the Red Giant Branch: Ambiguity in the Surface Boundary Condition Leads to ~100 K Uncertainty in Model Effective Temperatures
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• Age Determination of Galaxy Merger Remnant Stars using Asteroseismology
• Chromosome maps of Globular Clusters from wide-field ground-based photometry
• On the impact of the structural surface effect on global stellar properties and asteroseismic analyses
• Star Cluster Ages in the Gaia Era
• Improved calibration of the radii of cool stars based on 3D simulations of convection: implications for the solar model
• A Unified Exploration of the Chronology of the Galaxy
• Stellar evolution models with entropy-calibrated mixing-length parameter: application to red giants
• Connecting integrated RGB mass loss from asteroseismology and globular clusters
• Red giants evolutionary status determination: the complete Kepler catalog
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• Constraining the helium-to-metal enrichment ratio $\Delta Y/\Delta Z$ from main sequence binary stars. Theoretical analysis of the accuracy and precision of the age and helium abundance estimates
• Testing the 3-equation Kuhfuss Convection Model using the Sun
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