Analytical modelling of period spacings across the HR diagram

TL;DR

This paper presents and validates an analytical model for stellar period spacings across the HR diagram, enabling improved inference of core properties and chemical gradients from asteroseismic data.

Contribution

It introduces a non-sinusoidal analytical expression for period spacings that accounts for buoyancy glitches, validated against stellar model data with and without glitches.

Findings

Accurately infers glitch position and width with less than 7% error.

Reproduces mixed-mode period spacings and reveals frequency dependence of coupling coefficient.

Suggests a method to estimate pure acoustic mode frequencies from mixed-mode data.

Abstract

The characterisation of stellar cores may be accomplished through the modelling of asteroseismic data from stars exhibiting either gravity-mode or mixed-mode pulsations, potentially shedding light on the physical processes responsible for the production, mixing, and segregation of chemical elements. In this work we validate against model data an analytical expression for the period spacing that will facilitate the inference of the properties of stellar cores, including the detection and characterisation of buoyancy glitches (strong chemical gradients). This asymptotically-based analytical expression is tested both in models with and without buoyancy glitches. It does not assume that glitches are small and, consequently, predicts non-sinusoidal glitch-induced period-spacing variations, as often seen in model and real data. We show that the glitch position and width inferred from the fitting of the analytical expression to model data consisting of pure gravity modes are in close agreement (typically better than 7$\%$ relative difference) with the properties measured directly from the stellar models. In the case of fitting mixed-mode model data, the same expression is shown to reproduce well the numerical results, when the glitch properties are known a priori. In addition, the fits performed to mixed-mode model data reveal a frequency dependence of the coupling coefficient, $q$, for a moderate-luminosity red-giant-branch model star. Finally, we find that fitting the analytical expression to the mixed-mode period spacings may provide a way to infer the frequencies of the pure acoustic dipole modes that would exist if no coupling took place between acoustic and gravity waves.