Magnetism in Cool Evolved Stars

• Main Author: Mikhov, Stefan Georgiev
• Other Authors: Laboratoire Univers et Particules de Montpellier (LUPM), Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS), Université Montpellier, Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, Sofia, Agnès Lebre, Renada Konstantinova-Antova
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2021
• Subjects: Stellar Magnetism; Cool Evolved Stars; Stellar Evolution; Magnétisme Stellaire; Étoiles Froides Évoluées; Évolution Stellaire; [PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph]; Ari; narval
• Online Access: https://theses.hal.science/tel-03599880; https://theses.hal.science/tel-03599880/document; https://theses.hal.science/tel-03599880/file/GEORGIEV_2021_archivage.pdf

This Thesis presents a study of five magnetically active cool evolved stars of different spectral types and different evolutionary status. All the stars are studied using long-term monitoring in high resolution spectropolarimetry obtained with the instrument Narval and its successor, NeoNarval, installed on the Télescope Bernard Lyot at the Observatoire du Pic du Midi, France. The studied stars are three M giants (RZ Ari, β Peg, EK Boo), located on the tip-RGB and early-AGB, one post-AGB pulsating variable (R Sct) and one red supergiant (α Ori, a.k.a. Betelgeuse). To extract the mean polarization features from the stellar spectra, the Least Square Deconvolution (LSD) method is used.For the M giant stars, the surface magnetic field is detected and its longitudinal component (Bl) is directly measured from circular polarization observations (Stokes V). The temporal variability of the surface magnetic field is compared to that of spectral lines - indicators of magnetic activity: Ca H&K, Hα and the Ca infrared triplet lines. Based on the variability of Bl and the spectral activity indicators, the characteristics of the respective magnetic fields are inferred in the three M giants. An original improvement of the classical method for the determination of stellar parameters, including the projected rotational velocity vsini and the macroturbulent velocity vmac, is developed. This new method which combines spectrum synthesis and multi-line tools (LSD) is then used in order to determine the parameters vsini and vmac for the M giant RZ Ari. A more precise vsini will help for a better understanding of the origin of its surface magnetic field. For RZ Ari, a planet engulfment event is suspected to have triggered its magnetic activity. Another possible explanation for the magnetic field in M giants could be a local dynamo due to large convective cells, if such cells exist there. For β Peg and EK Boo, a possible explanation for the origin of the magnetic field is the α-Ω dynamo.For the post-AGB pulsating variable star R ...