Letter to the Editor

Extra modes in helium-core-burning stars probing an infra core cavity

¹ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université 92190 Meudon, France
² Department of Astronomy, School of Science, The University of Tokyo 7–3–1 Hongo Bunkyo-ku Tokyo 113–0033, Japan
³ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale 91405 Orsay, France
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP Rua das Estrelas 4150-762 Porto, Portugal
⁵ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire CS 34229 06304 Nice Cedex 4, France
⁶ IRAP, Université de Toulouse, CNRS, CNES, UPS 31400 Toulouse, France
⁷ Institut Universitaire de France (IUF)
⁸ Department of Physics & Astronomy ‘Augusto Righi’, University of Bologna Via Gobetti 93/2 40129 Bologna, Italy
⁹ INAF-Astrophysics and Space Science Observatory of Bologna Via Gobetti 93/3 40129 Bologna, Italy

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

Received: 1 December 2025
Accepted: 31 December 2025

Abstract

Context. Dipole mixed modes observed in the oscillation pattern of red giant stars probe the radiative regions in the stellar core.

Aims. Oscillation spectra of helium-core-burning stars sometimes show extra peaks that remain unexplained by the dipole mixed-mode pattern expected from the coupling of a radiative cavity in the stellar core and a pressure cavity in the stellar envelope.

Methods. We use the asymptotic expansion developed for a multi-cavity star in order to characterize these extra peaks.

Results. The analytical resonance condition of the multi-cavity gravito-acoustic modes, with two inner gravity cavities and an outer pressure cavity, helps us explain that the apparent extra peaks are dipole mixed modes that follow the three-cavity oscillation pattern. The derivation of the two asymptotic period spacings associated with the two distinct regions in the radiative core provides an estimate of the full radiative cavity.

Conclusions. Our results provide new constraints for analyzing the overshoot or mixing in the core of helium-core-burning stars. An important structure discontinuity inside the radiative core may explain the larger than expected observed period spacings.