Modelling Kepler red giants in eclipsing binaries: calibrating the mixing-length parameter with asteroseismology

Abstract

Stellar models rely on a number of free parameters. High-quality observations\nof eclipsing binary stars observed by Kepler offer a great opportunity to\ncalibrate model parameters for evolved stars. Our study focuses on six Kepler\nred giants with the goal of calibrating the mixing-length parameter of\nconvection as well as the asteroseismic surface term in models. We introduce a\nnew method to improve the identification of oscillation modes which exploits\ntheoretical frequencies to guide the mode identification ('peak-bagging') stage\nof the data analysis. Our results indicate that the convective mixing-length\nparameter (alpha) is about 14% larger for red giants than for the Sun, in\nagreement with recent results from modelling the APOGEE stars. We found that\nthe asteroseismic surface term (i.e. the frequency offset between the observed\nand predicted modes) correlates with stellar parameters (Teff, log g) and the\nmixing-length parameter. This frequency offset generally decreases as giants\nevolve. The two coefficients a_-1 and a_3 for the inverse and cubic terms that\nhave been used to describe the surface term correction are found to correlate\nlinearly. The effect of the surface term is also seen in the p-g mixed modes,\nhowever, established methods for correcting the effect are not able to properly\ncorrect the g-dominated modes in late evolved stars.\n