Modelling Kepler Red Giants in Eclipsing Binaries:Calibrating the Mixing Length Parameter with Asteroseismology

TL;DR

This study calibrates the mixing-length parameter in stellar models of Kepler red giants using a new mode identification method, revealing it is about 14% larger than in the Sun and exploring the surface term's dependence on stellar parameters.

Contribution

Introduces a novel mode identification technique guided by theoretical frequencies and calibrates the mixing-length parameter for red giants using Kepler data.

Findings

The mixing-length parameter (alpha) is approximately 14% larger in red giants than in the Sun.

The asteroseismic surface term correlates with stellar temperature, gravity, and mixing-length parameter.

The surface term decreases as giants evolve, but current correction methods struggle with g-dominated modes.

Abstract

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