Testing the entropy calibration of the radii of cool stars: models of alpha Centauri A and B

TL;DR

This paper applies an entropy calibration method to models of alpha Centauri A and B, achieving highly accurate stellar radii predictions without ad-hoc parameter tuning, improving stellar modeling precision.

Contribution

The study extends the entropy calibration technique to binary star models, demonstrating improved radius predictions for alpha Centauri A and B over traditional methods.

Findings

Models match observed radii within 1% accuracy.

Entropy calibration reduces reliance on ad-hoc alpha_MLT tuning.

Method achieves comparable or better accuracy than traditional models.

Abstract

We present models of alpha Centauri A and B implementing an entropy calibration of the mixing-length parameter alpha_MLT, recently developed and successfully applied to the Sun (Spada et al. 2018, ApJ, 869, 135). In this technique the value of alpha_MLT in the 1D stellar evolution code is calibrated to match the adiabatic specific entropy derived from 3D radiation-hydrodynamics simulations of stellar convective envelopes, whose effective temperature, surface gravity, and metallicity are selected consistently along the evolutionary track. The customary treatment of convection in stellar evolution models relies on a constant, solar-calibrated alpha_MLT. There is, however, mounting evidence that this procedure does not reproduce the observed radii of cool stars satisfactorily. For instance, modelling alpha Cen A and B requires an ad-hoc tuning of alpha_MLT to distinct, non-solar values. The entropy-calibrated models of alpha Cen A and B reproduce their observed radii within 1% (or better) without externally adjusted parameters. The fit is of comparable quality to that of models with freely adjusted alpha_MLT for alpha Cen B (within 1 sigma), while it is less satisfactory for alpha Cen A (within ~ 2.5 sigma). This level of accuracy is consistent with the intrinsic uncertainties of the method. Our results demonstrate the capability of the entropy calibration method to produce stellar models with radii accurate within 1%. This is especially relevant in characterising exoplanet-host stars and their planetary systems accurately.