Physical interpretation of the oscillation spectrum on the RGB and AGB

Abstract

The high-frequency resolution of the four-year $\text{Kepler}$ time series allows detailed study of seismic modes in luminous giants. Seismic observables help infer interior structures via comparisons with stellar models. We aim to investigate differences between H-shell (Red-Giant Branch; RGB) and He-burning (red clump and Asymptotic-Giant Branch; AGB) stars in the He-II ionisation zone and the sensitivity of seismic parameters to input physics in stellar models. We used a grid of stellar models with masses $0.8-2.5M_{\odot }$ and metallicities $-1.0-0.25$dex, including mass loss, overshooting, thermohaline mixing, and rotation-induced mixing. P-mode frequencies were inferred by suppressing g-modes in the core. The main factors affecting seismic observables are stellar mass and metallicity. The He-II glitch amplitude in the local large frequency separation $\Delta \nu$ correlates with the He-II ionisation zone density, explaining observed differences between RGB and clump/AGB stars. That amplitude exceeds 10% of $\Delta \nu$ in high-luminosity giants, making the asymptotic expansion less accurate when $\Delta \nu \le 0.5\mu$Hz. Mass loss on the RGB and rotation-induced mixing from the main sequence to the early-AGB produce phase differences in the He-II glitch modulation signature between RGB and clump/AGB stars. Efficient RGB mass loss (for $M\le 1.5M_{\odot }$) and mixing processes (for $M\ge 1.5M_{\odot }$) leave detectable signatures in p-mode frequencies, enabling classification of red giants.

21 pages, 12 figures (8 in main text, 4 in appendices)

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