Mixed-mode coupling in the Red Clump I. Standard single star models

TL;DR

This study explores how the coupling between oscillation modes in red giant stars depends on stellar properties, revealing correlations with mass and metallicity, and suggesting new methods to probe stellar internal structures.

Contribution

It provides a detailed analysis of mode coupling in core-helium burning stars using MESA models and analytical expressions, highlighting how coupling varies with stellar parameters and frequency.

Findings

Anti-correlation between coupling coefficient and stellar mass and metallicity.

Strongest coupling in red-horizontal branch and higher-mass stars.

Frequency dependence of coupling coefficient as a tool for internal stratification.

Abstract

The investigation of global, resonant oscillation modes in red giant stars offers valuable insights into their internal structures. In this study, we investigate in detail the information we can recover on the structural properties of core-helium burning (CHeB) stars by examining how the coupling between gravity- and pressure-mode cavities depends on several stellar properties, including mass, chemical composition, and evolutionary state. Using the structure of models computed with the stellar evolution code MESA we calculate the coupling coefficient implementing analytical expressions, which are appropriate for the strong coupling regime and the structure of the evanescent region in CHeB stars. Our analysis reveals a notable anti-correlation between the coupling coefficient and both the mass and metallicity of stars in the regime $M \lesssim 1.8~\mathrm{M}_\odot$, in agreement with Kepler data. We attribute this correlation primarily to variations in the density contrast between the stellar envelope and core. The strongest coupling is expected thus for red-horizontal branch stars, partially stripped stars, and stars in the higher-mass range exhibiting solar-like oscillations ($M \gtrsim 1.8~\mathrm{M}_\odot$). While our investigation emphasises some limitations of current analytical expressions, it also presents promising avenues. The frequency dependence of the coupling coefficient emerges as a potential tool for reconstructing the detailed stratification of the evanescent region.