Disks and winds around hot stars: new insights from multi-wavelength spectroscopy and interferometry

Abstract

Hot stars are the main source of ionization of the interstellar medium and its enrichment due to heavy elements. Constraining the physical conditions of their environments is crucial to understand how these stars evolve and their impact on the evolution of galaxies. Spectroscopy allows to access the physics, the chemistry, and the dynamics of these objects, but not the spatial distribution of these objects. Only long-baseline interferometry can resolve photospheres and close environments, and, combining spectroscopy and interferometry, spectro-interferometry allows to draw an even more detailed picture of hot stars. The objective of my thesis was to investigate the physical properties of the photosphere and circumstellar environment of massive hot stars confronting multi-band spectroscopic or spectro-interferometric observations and sophisticated non-LTE radiative transfer codes. My work was focused on two main lines of research. The first concerns radiative line-driven winds. Using UV and visible spectroscopic data and the radiative transfer code CMFGEN, I investigated the weak wind phenomenon on a sample of nine Galactic O stars. This study shows for the first time that the weak wind phenomenon, originally found for O dwarfs, also exists on more evolved O stars and that future studies must evaluate its impact on the evolution of massive stars. My other line of research concerns the study of classical Be stars, the fastest rotators among the non-degenerated stars, and which are surrounded by rotating equatorial disks. I studied the Be star o Aquarii using Hα (CHARA/VEGA) and Brγ (VLTI/AMBER) spectro-interferometric observations, the radiative transfer code HDUST, and developing new automatic procedures to better constrain the kinematics of the disk. This multi-band study allowed to draw the most detailed picture of this object and its environment, to test the limits of the current generation of radiative transfer models, and paved the way to my future work on a large samples of Be stars observed with VEGA, AMBER, and the newly available VLTI mid-infrared combiner MATISSE.