Study of magnetic fields in massive stars and intermediate-mass stars

• Main Author: Blazère, Aurore
• Other Authors: Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA (UMR_8109)), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Université Paris sciences et lettres, Coralie Laurence Neiner, Pascal Petit
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2016
• Subjects: Magnetic fields; Spectropolarimetry; Hot stars; Champs magnétiques; Spectropolarimetrie; Étoiles chaudes; [SDU.ASTR]Sciences of the Universe [physics]/Astrophysics [astro-ph]; Sirius; narval
• Online Access: https://theses.hal.science/tel-01484821; https://theses.hal.science/tel-01484821/document; https://theses.hal.science/tel-01484821/file/blazere_2016.pdf

Magnetic fields are known to play a fundamental role in stellar evolution but the magnetic properties of massive and intermediate-mass stars are not well understood. Only a small (7%) fraction of massive and intermediate-mass stars are found to be magnetic and their dipolar magnetic field strength is above ~300 Gauss. The current paradigm, the fossil field theory, describes this magnetism as remnant of an early phase of the star-life, but leaves many basic questions unanswered, such as the small fraction of magnetic stars, and in practice provides no constraint to stellar evolution theory. Recently, an ultra weak magnetic field (less than 1 Gauss) has been discovered in two intermediate mass stars (Vega and Sirius). They may be the first detections of a new type of weak magnetic fields. Two families of magnetic stars may thus exist: with strong or ultra-weak fields, separated by the so-called magnetic desert. My PhD thesis consists in analyzing observational data taken with high-resolution spectropolarimeters, mainly with Narval installed on the 2-meter telescope at the Pic du Midi Observatory, to detect magnetic fields. One part of my thesis is dedicated to the study of the weakest end of strong magnetic fields. I analyzed the observations of a massive O star, zeta Ori A. Only a few O stars are known to be magnetic and zeta Ori A has the weakest field. I was also involved in a project to determine the upper limit of the magnetic desert thanks to observations of Ap/Bp stars. The goal of these studies is to test the dependence of the upper limit with rotation and mass. The other part of my thesis is dedicated to the search for ultra-weak fields in hot stars to provide constraints to the various scenarios that explain the strong vs weak field dichotomy and improve our understanding of the properties of this kind of weak field. I present the result of the studies of normal stars, UZ Lyn and Vega, and of several chemically peculiar (Am and HgMn) stars. The studies presented in my PhD thesis provide new clues about ...