Dynamos of cool stars: Guiding theory to advanced models with spectropolarimetric observations.

PhD Bonnie ZAIRE

Abstract:

In the past decades, our knowledge of the magnetism of cool stars has largely grown thanks to tomographic imaging techniques that allow reconstructing large-scale surface fields of stars. The collection of maps obtained for stars with different spectral classes, rotating rates, and age unveiled a link between the complexity of the large-scale magnetic topology and the star’s internal structure. The large-scale magnetic field evolution is thought of as indirect observational evidence of dynamo action adjustments throughout the stellar evolution. Thus, the study of stellar magnetic fields has been closely associated with theoretical analysis of the magnetic field generation inside stars. In particular, magnetohydrodynamic numerical simulations have been performed in 3D to study how rotating turbulent convection generates and sustains magnetic fields.

This thesis is dedicated to the study of the magnetic morphology of cool stars. We extend the sample of stars with reconstructed large-scale magnetic fields by studying the active companion of the close-binary system V471 Tau: a K2 dwarf main-sequence star that is fastly rotating with a period of about 0.5 days. We use the information acquired to discuss the feasibility of some scenarios proposed to explain the eclipse timing variations observed in V471 Tau. Further, guided by the wealth of information provided by observations of the magnetic morphology of stars, we perform dynamo simulations to inquire about the possible parameters controlling the magnetic morphology of stars. Based on our parametric studies, we propose an energy ratio proxy that seems able to consistently classify the magnetic field complexity of early-M dwarfs as mainly dipolar or mainly multipolar.

The jury is composed by:

• Allan Sacha BRUN (Directeur de recherche CEA/Saclay — Rapporteur)
• Gaitee HUSSAIN (Chercheuse senior ESA — Rapporteure)
• Jérôme BOUVIER (Directeur de recherche CNRS — Examinateur)
• Michel RIEUTORD (Professeur UPS — Examinateur)
• Silvia ALENCAR (Professeur associé UFMG — Examinatrice)
• Matthew BROWNING (Professeur associé University of Exeter — Examinateur)
• Jean-François DONATI (Directeur de recherche CNRS — Directeur de thèse)
• Laurene JOUVE (Maitre de conference UPS — Co-directrice de thèse)

In the Coriolis room at OMP (IRAP side Belin).