Thèse Recherche de Champ Magnétique dans les Étoiles Géantes Brûlant de l'Helium au Coeur par l'Astérosismologie H/F

Doctorat.Gouv.Fr

The goal of this PhD thesis will be to search for internal magnetic fields in core-helium burning giants. For this purpose, the PhD candidate will first investigate the expected signatures of internal magnetic fields in core-helium burning giants using the formalism developed in Li et al. (2022). The magnetic field strength at this evolutionary stage will be estimated in two ways: by evolving stellar models starting from the fields that were measured in the hydrogen-burning phase (for primary clump stars), and by exploring the hypothesis that these fields originate from dynamo action in main-sequence convective cores. The signatures of magnetic fields will then be searched for using Kepler data, and on the longer term PLATO data. This will require to develop tools to analyse oscillation spectra for core-helium burning giants in an automatic way. The main detectable effect of core magnetic fields in these stars is the modification of the regular period spacing of gravity modes. One major complication for core-helium burning giants is the presence of so-called buoyancy glitches', which consist in near-discontinuities in the Brunt-Väisälä frequency profile (Vrard et al. 2022, Mosser et al. 2026). They arise near the boundary of the helium convective core and are also capable of modifying the period spacing of gravity modes. Consequently, the seismic search for magnetic fields in core-helium burning giants will require to also investigate buoyancy glitches in these stars in order to discriminate between the two effects.

If magnetic fields are detected in core-helium burning giants, valuable information will be obtained about their origin. Non-detections would also be interesting. For instance, secondary clump stars had large convective cores during the main sequence, in which magnetic fields are expected to be generated by dynamo. Failure to detect such fields in the core-helium burning phase would mean that magnetic fields are destroyed in the short-lived hydrogen-shell-burning in these stars, or that the field topology is unfavorable to detection.

Voir premier paragraphe du résumé. - Estimer l'intensité des champs magnétiques attendus au coeur des étoiles géantes brûlant de l'helium au coeur.
- Mettre au point des méthodes d'analyse sismique permettant de recherche de tels champs dans les étoiles du clump avec les données Kepler et PLATO.
- Interpréter les résultats en terme d'origine et d'évolution des champs magnétiques stellaires
- Modélisation stellaire (MESA)
- Analyse de donnée sismique