Physical interpretation of the oscillation spectrum on the RGB and AGB
Résumé
Context. The high frequency resolution of the four-year time series collected by the space-borne telescope Kepler gives us an opportunity to study the seismic mode structure of highly luminous giants in great detail. Seismic observables can be used as 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 heliumsecond ionisation zone, through the signature this zone leaves in mode frequencies. In addition, we explore the sensitivity of seismic parameters to the physics implemented in models. Methods. We used a grid of stellar models with mass between 0.8 M⊙ and 2.5 M⊙ and metallicity between -1.0 dex and 0.25 dex. Transfer mechanisms are implemented such as mass loss, core and envelope overshooting, and thermohaline mixing. We infer 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 of the helium ionisation zone glitch signature between H-shell and He-burning stars. The amplitude of the glitch signature in the local large separation, ∆ν, is correlated with the density in the helium ionisation zone, which explains the different glitch amplitudes observed between H-shell and He-burning stars. The amplitude exceeds 10% of the observed value of ∆ν in high-luminosity red giants, which makes the asymptotic expansion less accurate when ∆ν ≤ 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.
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