Evolution of Dipolar Mixed-mode Coupling Factor in Red Giant Stars: Impact of Buoyancy Spike

TL;DR

This study investigates how the buoyancy spike affects mixed-mode coupling in red giant stars, revealing that its impact varies with stellar structure and must be properly modeled for accurate frequency analysis.

Contribution

It demonstrates the influence of the buoyancy spike on the coupling factor q and shows how to reconcile asymptotic formalisms with observations by accounting for the glitch.

Findings

The applicability of asymptotic formalisms for q depends on the evanescent zone location.

Significant deviations in q occur in models with certain frequency separations and evanescent zone thickness.

Properly modeling the glitch improves the accuracy of inferred mode coupling factors.

Abstract

Mixed modes observed in red giants allow for investigation of the stellar interior structures. One important feature in these structures is the buoyancy spike caused by the discontinuity of the chemical gradient left behind during the first dredge-up. The buoyancy spike emerges at the base of the convective zone in low-luminosity red giants and later becomes a glitch when the g-mode cavity expands to encompass the spike. Here, we study the impact of the buoyancy spike on the dipolar mixed modes using stellar models with different properties. We find that the applicability of the asymptotic formalisms for the coupling factor, q, varies depending on the location of the evanescent zone, relative to the position of the spike. Significant deviations between the value of q inferred from fitting the oscillation frequencies and either of the formalisms proposed in the literature are found in models with a large frequency separation in the interval 5 to 15 microHz, with evanescent zones located in a transition region which may be thin or thick. However, it is still possible to reconcile q with the predictions from the asymptotic formalisms, by choosing which formalism to use according to the value of q. For stars approaching the luminosity bump, the buoyancy spike becomes a glitch and strongly affects the mode frequencies. Fitting the frequencies without accounting for the glitch leads to unphysical variations in the inferred q, but we show that this is corrected when properly accounting for the glitch in the fitting.