Studies of nebulae ejected from massive stars based on observations with the Herschel space observatory

This PhD thesis is devoted to the study of the nebulae ejected by the luminous blue variable (LBV) stars WRAY 15-751 and AG Car as well as by the Wolf-Rayet star WR 124. It is based on new infrared data taken by the Herschel Space Observatory. These data were complemented by optical data as well as by archived infrared observations. In the first chapter, a general introduction to the subject is given along with a description of the methods used for the data analysis. The three following chapters contain the study of each one of the three nebulae separately. This led to the determination of the physical parameters of the dust and gas components, with the aim to shed a new light on the mass-loss history of the central stars. In particular, the new Herschel infrared images provide a detailed mapping of the nebular dust distribution and of the circumstellar environment at different scales, revealing multiple shells and cavities linked to previous mass-loss events. Moreover, the combination of Herschel photometric results along with archival data was used to model dust with the help of a two-dimensional publicly available radiative transfer code. This model provided us with the dust mass, temperature and composition. The Herschel infrared spectra of all nebulae revealed forbidden nebular emission lines coming not only from an ionized gas region but also from a region where the gas is neutral. Based on the emission line flux measurements, the gas mass and the abundance ratios were estimated. These results, combined with the theoretical models of stellar evolution, were then used to constrain the evolutionary stage of the star at which each nebula was ejected. This study underlines the importance of the Herschel infrared observations for these nebulae. Thanks to them, a better understanding of these objects has been achieved. In particular, a second bigger and fainter dust shell was discovered around WRAY 15-751. In addition, all nebulae were found lying in empty cavities, probably formed during a previous evolutionary phase of their central star. The dust models indicated the necessity to include large grains in all nebulae to reproduce the data. This study showed that for the LBV WRAY 15- 751, with an initial mass of 40 M sun, the nebular ejection took place during a red supergiant phase, while for the LBV AG Car, with an initial stellar mass of 55 M sun, the ejection happened during a cool LBV phase. For the nebula M1-67 around the star WR 124, with an initial mass of 60 M sun, the ejection also took place during a LBV phase. These results are in agreement with the current evolutionary models with little stellar rotation