Contributions of structural variations to the asymptotic mixed-mode coupling Factor in red giant stars

TL;DR

This paper investigates how structural variations, especially chemical discontinuities, influence the asymptotic mixed-mode coupling factor in red giant stars, revealing their impact on observable stellar oscillation properties.

Contribution

It provides a detailed analysis of how chemical composition discontinuities affect the mixed-mode coupling factor using stellar models, highlighting the role of structural variations.

Findings

Structural variations cause subtle shifts in Lamb frequency and sharp changes in buoyancy frequency.

These variations lead to detectable changes in the mixed-mode coupling factor in low-luminosity red giants.

A sudden increase and subsequent drop in the coupling factor are observed as stars evolve along the red giant branch.

Abstract

The advent of ultra-precise photometry space missions enable the possibility of investigating stellar interior with mixed modes. The structural variations induced by the discontinuity of the chemical composition left behind during the first dredge--up is an important feature in the stellar mid-layers located between the hydrogen-burning shell and the base of the convective zone of red giants, as the mixed-mode properties can be significantly affected by these variations. In this paper, the contributing factors to variations of the mixed-mode coupling factor, $q$, are discussed with stellar models. In general, the structural variations give rise to a subtle displacement in the Lamb frequency and a sharp change in the buoyancy frequency, which lead to variations in the value of $q$ computed using the asymptotic formalisms that assuming a smooth background free of structural variations. The impact of these two factors can be felt in detectable mixed modes in low-luminosity red giants. Furthermore, the different nature of variations of the two characteristic frequencies with radius near the base of the convective zone, produces a sudden increase in $q$ in evolved red giants. This is followed by a quick drop in $q$ as the star evolves further along the red giant branch.