Structural glitches near the cores of red giants revealed by oscillations in g-mode period spacings from stellar models

TL;DR

This paper investigates how core structural glitches in red giants influence their oscillation patterns, providing analytical tools and stellar model analysis to identify these features and their evolutionary significance.

Contribution

It introduces an analytical expression for oscillation patterns affected by core glitches and demonstrates their impact on seismic diagnostics during red giant evolution stages.

Findings

Glitches cause observable variations in period spacings and mode inertias.

Significant changes in mode amplitude and frequency patterns occur at specific evolutionary stages.

Glitch effects can lead to misinterpretation of stellar properties if neglected.

Abstract

With recent advances in asteroseismology it is now possible to peer into the cores of red giants, potentially providing a way to study processes such as nuclear burning and mixing through their imprint as sharp structural variations -- glitches -- in the stellar cores. Here we show how such core glitches can affect the oscillations we observe in red giants. We derive an analytical expression describing the expected frequency pattern in the presence of a glitch. This formulation also accounts for the coupling between acoustic and gravity waves. From an extensive set of canonical stellar models we find glitch-induced variation in the period spacing and inertia of non-radial modes during several phases of red-giant evolution. Significant changes are seen in the appearance of mode amplitude and frequency patterns in asteroseismic diagrams such as the power spectrum and the \'echelle diagram. Interestingly, along the red-giant branch glitch-induced variation occurs only at the luminosity bump, potentially providing a direct seismic indicator of stars in that particular evolution stage. Similarly, we find the variation at only certain post-helium-ignition evolution stages, namely, in the early phases of helium-core burning and at the beginning of helium-shell burning signifying the asymptotic-giant-branch bump. Based on our results, we note that assuming stars to be glitch-free, while they are not, can result in an incorrect estimate of the period spacing. We further note that including diffusion and mixing beyond classical Schwarzschild, could affect the characteristics of the glitches, potentially providing a way to study these physical processes.