Abstract :
[en] 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.
