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

Établissement : Université de Toulouse École doctorale : SDU2E - Sciences de l'Univers, de l'Environnement et de l'Espace Laboratoire de recherche : IRAP - Institut de Recherche en Astrophysique et Planetologie Direction de la thèse : Sébastien DEHEUVELS ORCID 0000000343175460 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-06-01T23:59:59 Magnetic fields play a crucial role in stellar evolution. By reshaping the internal rotation, they can control the redistribution of chemical elements in radiative layers, and thus the evolution of stars. While magnetic fields have been measured at the surface of stars across the HR diagram, the opacity of stellar matter had until recently hidden internal fields to the observations. In Li et al. 2022 (https://rdcu.be/cWXAb), we obtained the first direct measurement of magnetic fields in red giant cores using asteroseismology, which is the study of waves propagating inside stars to probe their interiors. Oscillation modes in red giants have a mixed nature, behaving both as pressure modes in the envelope and as gravity modes in the core. Magnetic fields affect oscillation modes by inducing a characteristic shift in their frequencies. In our recent studies, we detected such shifts in 80 red giant branch stars (i.e., hydrogen-shell-burning giants) using data from the Kepler satellite (NASA). While this is an unprecedented achievement, this represents only a small fraction of the several thousands of red giants with detected oscillations (~ 20,000). Also, about half of these stars are core-helium burning giants, in which magnetic fields have not yet been searched for. Core-helium burning giants with masses below about 2 Msun (also know as 'primary clump' stars) are the descendants of red giant branch stars. Those that have higher masses (secondary clump' stars) trigger helium-burning in the core without ascending the red giant branch.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

Master 2 recherche en astrophysique incluant un cours en physique stellaire. Des bases en astérosismologie seront appréciées mais pas obligatoires.