Aim. In this project, theoretical models of dust driven winds of asymptotic giant branch (AGB) stars with effective temperatures within a range of 2400 − 3200 [K] and relative carbon-to-oxygen abundance C/O > 1 are studied. The aim is to understand if and how a detailed description of the grain size in winds of carbon rich AGB stars affects the wind formation and wind driving processes. Method. The computations were performed with a well tested FORTRAN code by calculating a grid of 60 models with different stellar parameters using grain size-dependent opacities. The results were then compared with models where the small particle approximation (SPA) had been used. Conclusions. The results indicate a certain dependency on grain size of the wind properties. The results from the computations of the majority of the models show no significant diferences however, especially not for the mass loss rates. Thus earlier computations performed using the SPA need not necessarily to be rejected.

Large spectroscopic surveys are trailblazing endeavours in the study of stellar archaeology and near eld cosmology. Access to homogeneous databases of thousands of stellar spectra allow for a detailed and statistically satisfying look into the chemical abundance distribution of our Galaxy and its surrounding satellites, ultimately working towards a better understanding of galactic chemical evolution. This thesis presents the work of three new studies at the current frontier of stellar archaeology. Through the rst look at carbon-enhanced metal-poor (CEMP) stars using H-band spectra, six new CEMP stars and another seven likely candidates were found within the APOGEE database following Data Release 12. These stars have chemical compositions typical of metal-poor halo stars, however the alpha-abundances of two stars indicate possible origins in an accreted dwarf galaxy. A lack of heavy element spectral lines impedes further sub-classi cation of these CEMP stars, however, based on radial velocity scatter, we predict most are not CEMP-s stars which are typically found in binary systems. This preliminary investigation warrants optical observations to con rm the stellar parameters and low metallicities of these stars, to determine the heavy-element abundance ratios and improve the precision in the derived abundances, and to examine their CEMP sub-classi cations. Additionally, the rst results for the spectroscopic follow up to the Pristine survey are presented. Using a sample of 149 stars, a success rate of 70% for finding stars with [Fe/H]<-2.5 and 22% for finding stars with [Fe/H]<-3.0 is reported, significantly higher than other surveys that typically report success rates of 3-4% for recovering stars with [Fe/H]<-3.0. Finally, the new spectral analysis tool StarNet is introduced. A deep neural network architecture is used to examine both synthetic stellar spectra and SDSS-III APOGEE spectral data and can produce the stellar parameters of temperature, gravity, and metallicity with similar or better precision as the APOGEE pipeline values when trained directly with the APOGEE spectra. StarNet is capable of being trained on synthetic data as well, and is able to reproduce the stellar parameters for both synthetic and APOGEE spectra, including low signal-to-noise spectra, with similar precision to training on the APOGEE spectra itself. The residuals between StarNet predictions and APOGEE DR13 parameters are similar to or better than the di erences between the APOGEE DR13 results and optical high resolution spectral analyses for a subset of benchmark stars. While developed using the APOGEE spectral database (real spectra and corresponding ASSET synthetic data with similar normalization functions), StarNet should be applicable to other large spectroscopic surveys like Pristine. / Graduate

Nous présentons les résultats de trois campagnes d'observation d'un mois chacune dans le cadre de l'étude de la collision des vents dans les systèmes binaires Wolf-Rayet + OB. Ce travail se concentre sur l'étude des objets de l'hémisphère sud n'ayant jamais encore fait l'objet d'études poussées dans ce contexte. À cela, nous avons ajouté l'objet archétype pour ce type de systèmes : WR 140 (WC7pd + O5.5fc) qui a effectué son dernier passage périastre en janvier 2009. Les deux premières campagnes (spectroscopiques), ont permis une mise à jour des éléments orbitaux ainsi qu'une estimation de la géométrie de la zone de collision des vents et d'autres paramètres fondamentaux des étoiles pour 6 systèmes binaires : WR 12 (WN8h), 21 (WN5o+O7V), 30 (WC6+O7.5V), 31 (WN4o+O8), 47 (WN6o+O5) et 140. Une période non-orbitale courte (probablement reliée à la rotation) a également été mesurée pour un des objets : WR 69 (WC9d+OB), avec une période orbitale bien plus grande. La troisième campagne (photométrique) a révélé une variabilité étonnamment faible dans un échantillon de 20 étoiles WC8/9. Cela supporte l'idée que les pulsations ne sont pas courantes dans ce type d'étoiles et qu'il est peu probable que celles-ci soient le mécanisme dominant de formation de poussière, suggérant, par défaut, le rôle prédominant de la collision des vents. / We present the results from three month-long observational campaigns related to WR+O colliding-wind binaries. This work focuses on southern-hemisphere systems that have not yet been previously studied in this context. To this we add the northern archetype colliding-wind system WR 140 (WC7pd + O5.5fc), which recently suffered another active periastron passage in 2009 January. The first two (spectroscopic) campaigns allowed us to update the orbital elements and to constrain the geometry of the colliding wind region and other fundamental parameters, for 6 binary systems: WR 12 (WN8h), WR 21 (WN5o+O7V), WR 30 (WC6+O7.5V), WR 31 (WN4o+O8), WR 47 (WN6o+O5) and WR 140. A short non-orbital (probably rotational) period was also detected in the WR component of one of our objects, WR 69 (WC9d+OB), with a much longer orbital period. The third (photometric) campaign revealed a surprisingly low level of variability in our sample of 20 WC8/9 stars. This supports the idea that pulsations are not very frequent in this type of star and are unlikely to be the main mechanism for dust formation, suggesting, by default, a predominant role of wind-wind collision.

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