Facing the phase: Gravity-mode offset and buoyancy glitches in red--giant branch stars

TL;DR

This paper investigates the physical origins of phase offsets in gravity modes of red-giant stars, revealing that buoyancy glitches significantly influence the observed frequency patterns and proposing a modified interpretative formula.

Contribution

It identifies the role of buoyancy glitches in g-mode phase interpretation and introduces a formalism to quantify their impact, enhancing theoretical understanding of stellar oscillation data.

Findings

Buoyancy glitches significantly affect g-mode phase interpretation.

A formalism to quantify glitch contributions to phase is developed.

Proposes a modification to the standard g-mode phase formula.

Abstract

With the increasing precision of asteroseismic observations, it becomes possible to reliably measure oscillation properties of an increasing number of stars. Interpreting these measurements requires a good theoretical understanding of their link to fundamental stellar properties. In this study, we focus on the phase offset in gravity(g)-mode frequencies, which is imprinted in the asymptotic eigenfrequency pattern of mixed dipole modes observed in red--giant branch stars. We aim to unravel its physical origin and thus enable an informed interpretation of observations. Using stellar models, we empirically test the contribution of the g-mode offset $\varepsilon_\mathrm{g}$ (which is related to the wave reflection at cavity boundaries and commonly considered to be the dominant phase term) and glitches to the total observable phase. We find that, additionally to $\varepsilon_\mathrm{g}$, buoyancy glitches play an important role in the correct interpretation of the g--mode frequency phase. We further find that glitches in the evanescent zone also contribute to the phase, and we present a formalism to quantify this contribution. Finally, we propose a modification to the widely used formula for $\varepsilon_\mathrm{g}$. The g--mode frequency phase carries more information than previously considered. It has large analytic potential to study not only the reflection properties of the buoyancy cavity, but also the properties of glitches in the Brunt-V\"ais\"al\"a frequency.