Physical interpretation of the oscillation spectrum on the red giant branch and the asymptotic giant branch

¹ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS 5 Place Jules Janssen 92190 Meudon, France
² School of Physics, University of New South Wales Sydney NSW 2052, Australia
³ Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251 F-35000 Rennes, France
⁴ Instytut Astronomiczny, Uniwersytet Wrocławski ul. Kopernika 11 51-622 Wroclaw, Poland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 February 2025
Accepted: 22 September 2025

Abstract

Context. The high frequency resolution of the four-year time series collected by the space-borne telescope Kepler provides us the opportunity to study the seismic mode structure of highly luminous giants in great detail. Seismic observables can be used to infer the interior structure through comparisons with stellar models. However, we still need to extend the physical interpretation of previously observed seismic differences between hydrogen-shell burning (red giant branch; RGB) and helium-burning (red clump and asymptotic giant branch; AGB) stars towards high luminosity stages.

Aims. Here we aim to investigate which physical conditions differ between H-shell and He-burning stars in the helium-second ionisation zone, based on the signature this zone imprints on mode frequencies. In addition, we explore the sensitivity of seismic parameters to the physics implemented in the models.

Methods. We used a grid of stellar models with masses between 0.8 M_⊙ and 2.5 M_⊙ and metallicities between −1.0 dex and 0.25 dex. Transfer mechanisms such as mass loss, core, envelope overshooting, and thermohaline mixing were implemented. We inferred the p-mode frequencies of the models by artificially suppressing the gravity modes in the core.

Results. In accordance with observations, we find that the main stellar properties affecting the seismic observables in the models are the stellar mass and metallicity. Mass loss on the RGB and rotation-induced mixing from the main sequence to the early-AGB cause a phase difference in the helium ionisation zone glitch signature between H-shell and He-burning stars. The amplitude of the glitch signature in the local large separation, Δν, correlates with the density in the helium ionisation zone, which explains the different glitch amplitudes observed between the H-shell and He-burning stars. The amplitude exceeds 10% of the observed value of Δν in high-luminosity red giants, which makes the asymptotic expansion less accurate when Δν ≤ 0.5 μHz.

Conclusions. An efficient mass loss on the RGB, typically encountered when M ≤ 1.5 M_⊙, can explain the classification of H-shell and He-burning stars based on the p-mode pattern. When M ≥ 1.5 M_⊙, efficient mixing mechanisms might leave an important detectable signature in the p-mode frequencies, permitting a potential classification of these stars.