Context. Irregular satellites of Jupiter with their highly eccentric, inclined and distant orbits suggest that their capture took place after the giant planet migration. Aims. We aim to improve our understanding of the surface composition of irregular satellites of Jupiter to gain insight into a narrow time window when our solar system was forming. Methods. We observed three Jovian irregular satellites, Himalia (JVI), Elara (JVII), and Carme (JXI), using a medium-resolution 0.8-5.5 mu m spectrograph, SpeX on the NASA Infrared Telescope Facility (IRTF). Using a linear spectral unmixing model we have constrained the major mineral phases on the surface of these three bodies. Results. Our results confirm that the surface of Himalia (JVI), Elara (JVII), and Carme (JXI) are dominated by opaque materials such as those seen in carbonaceous chondrite meteorites. Our spectral modeling of NIR spectra of Himalia and Elara confirm that their surface composition is the same and magnetite is the dominant mineral. A comparison of the spectral shape of Himalia with the two large main C-type asteroids, Themis (D similar to 176 km) and Europa (D similar to 352 km), suggests surface composition similar to Europa. The NIR spectrum of Carme exhibits blue slope up to 1.5 mu m and is spectrally distinct from those of Himalia and Elara. Our model suggests that it is compositionally similar to amorphous carbon. Conclusions. Himalia and Elara are compositionally similar but di ff er significantly from Carme. These results support the hypotheses that the Jupiter's irregular satellites are captured bodies that were subject to further breakup events and clustered as families based on their similar physical and surface compositions.

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

Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. Aims. We explore the photometric and spectroscopic behavior of ASASSN-13db during the 2014-2017 outburst. Methods. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the light curve of ASASSN-13db and the dynamical and physical properties of the accretion flow. Results. The 2014-2017 outburst lasted for nearly 800 days. A 4.15 d period in the light curve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blue-shifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of similar to 5800-6000 K in the accretion flow. Conclusions. Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (<(M)over dot> = 1-3 x 10(-7) M-circle dot/yr), about two orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very-low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settle after the beginning of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars.

Ce mémoire présente une exploitation additionnelle du catalogue LSPM comportant deux grandes parties, soit la poursuite du relevé des naines blanches à l'intérieur d'un rayon de 40 pc et l'étude de la contrepartie SDSS de ce catalogue. La première consiste à utiliser les critères basés sur des diagrammes de mouvements propres réduits établis dans des travaux antérieurs afin de dresser une liste de candidates naines blanches. Quelques modifications quant à l'ordre de priorité ainsi que le critère basé sur les magnitudes photographiques seront apportées. Une approche de moindres carrés non-linéaire appliquée aux magnitudes observées dans les systèmes photométriques disponibles permet ensuite de déterminer la distance des objets sélectionnés par comparaison avec les plus récents modèles de naines blanches disponibles. Un second critère est appliqué en se basant sur la qualité de l'ajustement résultant de la procédure. Ceci mènera à l'identification de 31 nouvelles naines blanches, dont 11 situées vraisemblablement à moins de 40 pc. Une nouvelle liste de candidates a aussi pu être établie pour la poursuite du relevé. La seconde partie consiste à utiliser tous les objets du 7e relevé du SDSS identifiés dans le catalogue LSPM pour l'étude statistique des naines blanches, essentiellement via la détermination de leur fonction de luminosité. Pour ce faire, un critère basé sur les diagrammes de mouvements propres réduits sera encore une fois utilisé. La distance des objets sélectionnés sera déterminée selon la même procédure, mais cette fois en ne se limitant pas à 40 pc. La méthode de pondération selon le volume observable (1/v_max) est utilisée afin de compenser pour le biais introduit par la sensibilité du catalogue à la magnitude. Cependant, d'autres facteurs viennent influencer la détermination de la fonction de luminosité et une analyse de ceux-ci est finalement présentée. / We present an additional exploitation of the Lépine \& Shara Proper Motion catalog (LSPM) divided in two main parts: a follow up and some improvements on the census or northern white dwarfs within 40 pc of the Sun and a study of the SDSS white dwarfs component in the LSPM survey. The former consists in the use of criteria previously established in order to create a list of white dwarf candidates with an associated priority. The priority order has been enriched and one of the criteria has been slightly modified. We then use a non-linear least square method to the observed magnitudes in each available photometric system simultaneously in order to determine the atmospheric parameters and, in particular, the distance of each white dwarf candidate. This approach allows a second criteria to be applied on our sample based on the goodness of the fit. This will lead us to the identification of 31 new white dwarfs, from which 11 are likely to remain within 40 pc of the Sun. A new list of 340 candidates has also been established for eventual observations. The latter consists in the use of all the objects from the 7th data release of the SDSS that have a counterpart in the LSPM catalog to elaborate a statistical study of white dwarfs, in this case by calculating the white dwarf luminosity function. To do so, one of the same criterion mentioned above will be used to make a first selection of presumed white dwarfs. Afterwards, the distance will be determined by the same least-square method, but without any restriction on the distance. To balance the effect due to the sensibility in magnitude of the survey, we used a ponderation method based on the maximum observable volume (1/v_max). However, other factors come to affect our results and the last part of this work concentrates on their identification.

Galaxy clusters are the largest gravitationally bound structures in the Universe. Their masses are dominated by dark matter ($\sim$85\% of the mass) with stars representing 1-4\% of their masses. A hot, X-ray emitting gas called the intracluster medium makes most of their baryonic mass. The presence of this gas and of numerous neighbouring galaxies prematurely stop the star formation in clusters. In other terms, more galaxies in clusters are passive than in the general population of galaxies. This effect is mass and position-dependant: high-mass galaxies are more likely to be passive than less massive ones; galaxies inhabiting the cluster core are also less likely to form stars than those in the outskirts. The fraction of passive galaxies is greater in local clusters than in high-redshift ones, because they had more time to evolve. Much is unknown about the cessation of star formation, called quenching, in clusters. Thus, although many examples of infalling galaxies being stripped of their gas have been reported for low-mass galaxies, it is unclear if the most massive members became quenched before or after they become cluster members. The relationship between quenching and the cluster mass is also poorly understood. Despite the variety of methods devised to find clusters of galaxies, most of what we know about quenching in $z\gtrsim 1$ clusters was discovered with optically/infrared-selected cluster samples (clusters found as overdensities of galaxies), or samples of mixed origin. Yet, there is tentative evidence that optically/infrared-selected samples are biased toward having more passive galaxies than those that were X-ray selected. In the present dissertation, quenching is explored in X-ray selected cluster samples. A sample of high-redshift, low-mass galaxy clusters is built by finding galaxy overdensities coincident with sources of extended X-ray emission. A photometry-based analysis reveals that the fraction of quenched galaxies in these clusters is very variable. Moreover, the brightest cluster galaxies are also diverse. Yet, for all the information that photometry can provide, this sample candidate clusters need to be confirmed with spectroscopy. Spectroscopic observations obtained for four candidate clusters are reduced and analysed. The results show that three of them are clusters, the fourth candidate being a superposition of structures. Member spectra are examined to infer their star formation history, and the results shows the existence of an intermediary population of galaxies, where an old stellar population coexists with weak star formation. Finally, the galaxies of a $z=1.98$ X-ray selected cluster, XLSSC 122 are investigated in detail. Photometric data in 12 bands are organized to perform spectral energy distribution fittings, a technique that allows a simplified reconstitution of the history of the star formation. Results show that the members were formed at diverse epochs, the oldest being about 2.5 Gyrs old. Simulations drawn from the Multi Dark Planck 2 are used to infer the mass-scale of the cluster when the oldest galaxies were formed, something that has never been done before. The oldest galaxies were probably formed when XLSSC 122 had accreted $<$10\% of its $z=1.98$ mass, i.e. the mass-scale of a galaxy group. / Graduate

De nouveaux modèles d'atmosphère sont présentés, incluant les profils de raie d'hélium neutre améliorés de Beauchamp (1995) et le formalisme de probabilité d'occupation pour ce même atome. Ces modèles sont utilisés pour calculer une grille de spectres synthétiques correspondant à des atmosphères riches en hélium et contenant des traces d'hydrogène. Cette grille est utilisée pour déterminer les paramètres atmosphériques principaux des étoiles de notre échantillon, soient la température effective, la gravité de surface et l'abondance d'hydrogène. Notre échantillon contient des spectres visibles de haut rapport signal-sur-bruit pour 102 naines blanches riches en hélium, dont 29 ont été observés au cours de ce projet, ce qui en fait le plus grand échantillon de spectres de qualité de naines blanches riches en hélium. Des spectres synthétiques ont été calculés en utilisant différentes valeurs du paramètre α de la théorie de la longueur de mélange dans le but de calibrer empiriquement la valeur de ce paramètre pour les DB. Afin d'améliorer la précision sur les paramètres atmosphériques de quelques étoiles, nous avons utilisé des spectres couvrant la raie Hα pour mieux déterminer l'abondance d'hydrogène. Finalement, nous avons calculé la distribution de masse de notre échantillon et la fonction de luminosité des DB. La distribution de masse montre une coupure à 0.5 fois la masse solaire qui est prédite par les modèles d'évolution stellaire et dévoile une masse moyenne significativement plus élevée pour les étoiles de type DBA. La masse moyenne de l'ensemble des DB et DBA est très proche de celle des DA. La fonction de luminosité nous permet de calculer que le rapport du nombre de DB sur le nombre de DA vaut environ 25%. / New model atmospheres are presented, including improved neutral helium lines from Beauchamp (1995) and the occupation probability formalism for that atom. These models are used to compute a grid of synthetic spectra for helium rich atmospheres with different hydrogen abundances. This grid is used to determine the principal atmospheric parameters of the stars in our sample, e.g. effective temperature, surface gravity and hydrogen abundance. There are 102 high quality spectra of helium-rich white dwarfs in our sample, making it the largest sample of this kind. 29 of these spectra were observed for this project. Synthetic spectra using different values of the α parameter from the mixing length theory have been calculated in order to determine the correct value of this parameter for DB model atmospheres. Finally, we have computed the mass distribution of our sample and the DB luminosity funtion. The mass distribution shows a clear cutoff at 0.5 solar masses which is predicted by stellar evolution theory and gives a significantly higher mean mass for the DBA stars of the sample. However, the global mean mass of our sample is very close to that of DA stars. With our luminosity function, we found a number ratio of DB stars over DA stars of about 25%.

Diverses méthodes ont été utilisées pour étudier les étoiles Wolf-Rayet (WR) dans le but de comprendre les phénomènes physiques variés qui prennent place dans leur vent dense. Pour étudier la variabilité qui n'est pas strictement périodique et ayant des caractéristiques différentes d'une époque à l'autre, il faut observer pendant des périodes de temps suffisamment longues en adopter un échantillonnage temporel élevé pour être en mesure d'identifier les phénomènes physiques sous-jacents. À l'été 2013, des astronomes professionnels et amateurs du monde entier ont contribué à une campagne d'observation de 4 mois, principalement en spectroscopie, mais aussi en photométrie, polarimétrie et en interférométrie, pour observer les 3 premières étoiles Wolf-Rayet découvertes: WR 134 (WN6b), WR 135 (WC8) et WR 137 (WC7pd + O9). Chacune de ces étoiles est intéressante à sa manière, chacune présentant une variété différente de structures dans son vent. Les données spectroscopiques de cette campagne ont été réduites et analysées pour l'étoile présumée simple WR 134 pour mieux comprendre le comportement de sa variabilité périodique à long terme dans le cadre d'une étude des régions d'interactions en corotation (CIRs) qui se retrouvent dans son vent. Les résultats de cette étude sont présentés dans ce mémoire. / Wolf-Rayet stars have been studied using various methods in order to understand the many physical phenomena taking place in their dense outflows. In the case of variability that is not strictly periodic or for epoch-dependant changes, the challenge is to observe for su fficiently long periods of time and with a high enough time sampling to be able to understand the underlying phenomena. During the summer of 2013, professional and amateur astronomers from around the world contributed to a 4-month campaign, mainly in spectroscopy but also in photometry, polarimetry and interferometry, to observe the fi rst 3 Wolf-Rayet stars discovered: WR 134 (WN6b), WR 135 (WC8) and WR 137 (WC7pd+O9). Each of these stars are interesting in their own way, showing a variety of stellar wind structures. The spectroscopic data from this campaign have been reduced and analyzed for the presumably single star WR 134 in order to better understand its behavior and long-term periodicity in the context of corotating interaction regions (CIRs) in the wind. The results of this study are presented in this thesis.

De nouveaux modèles d'atmosphère sont présentés, incluant les profils de raie d'hélium neutre améliorés de Beauchamp (1995) et le formalisme de probabilité d'occupation pour ce même atome. Ces modèles sont utilisés pour calculer une grille de spectres synthétiques correspondant à des atmosphères riches en hélium et contenant des traces d'hydrogène. Cette grille est utilisée pour déterminer les paramètres atmosphériques principaux des étoiles de notre échantillon, soient la température effective, la gravité de surface et l'abondance d'hydrogène. Notre échantillon contient des spectres visibles de haut rapport signal-sur-bruit pour 102 naines blanches riches en hélium, dont 29 ont été observés au cours de ce projet, ce qui en fait le plus grand échantillon de spectres de qualité de naines blanches riches en hélium. Des spectres synthétiques ont été calculés en utilisant différentes valeurs du paramètre α de la théorie de la longueur de mélange dans le but de calibrer empiriquement la valeur de ce paramètre pour les DB. Afin d'améliorer la précision sur les paramètres atmosphériques de quelques étoiles, nous avons utilisé des spectres couvrant la raie Hα pour mieux déterminer l'abondance d'hydrogène. Finalement, nous avons calculé la distribution de masse de notre échantillon et la fonction de luminosité des DB. La distribution de masse montre une coupure à 0.5 fois la masse solaire qui est prédite par les modèles d'évolution stellaire et dévoile une masse moyenne significativement plus élevée pour les étoiles de type DBA. La masse moyenne de l'ensemble des DB et DBA est très proche de celle des DA. La fonction de luminosité nous permet de calculer que le rapport du nombre de DB sur le nombre de DA vaut environ 25%. / New model atmospheres are presented, including improved neutral helium lines from Beauchamp (1995) and the occupation probability formalism for that atom. These models are used to compute a grid of synthetic spectra for helium rich atmospheres with different hydrogen abundances. This grid is used to determine the principal atmospheric parameters of the stars in our sample, e.g. effective temperature, surface gravity and hydrogen abundance. There are 102 high quality spectra of helium-rich white dwarfs in our sample, making it the largest sample of this kind. 29 of these spectra were observed for this project. Synthetic spectra using different values of the α parameter from the mixing length theory have been calculated in order to determine the correct value of this parameter for DB model atmospheres. Finally, we have computed the mass distribution of our sample and the DB luminosity funtion. The mass distribution shows a clear cutoff at 0.5 solar masses which is predicted by stellar evolution theory and gives a significantly higher mean mass for the DBA stars of the sample. However, the global mean mass of our sample is very close to that of DA stars. With our luminosity function, we found a number ratio of DB stars over DA stars of about 25%.

Detailed chemical abundances of globular clusters in the Milky Way and M31 (the Andromeda Galaxy) are presented based on analyses of high resolution spectra. The unusual Milky Way cluster Palomar 1 (Pal 1) is studied through spectra of individual red giant branch stars; these abundances show that Pal 1 is not a classical globular cluster, and may have been accreted from a dwarf satellite of the Milky Way. The Milky Way globular clusters 47 Tuc, M3, M13, NGC 7006, and M15 are studied through their integrated light (i.e. a single spectrum is obtained for each cluster) in order to test high resolution integrated light analyses. The integrated abundances from these clusters reproduce the average abundances from individual stellar analyses for elements that do not vary within a cluster (e.g. Fe, Ca, and Ni). For elements that do vary within the clusters (e.g. Na and Mg) the integrated abundances fall within the observed ranges from individual stars. Certain abundance ratios are found to be extremely sensitive to uncertainties in the underlying stellar populations, such as input models, empirical relations to determine atmospheric parameters, interloping field stars, etc., while others (such as [Ca I/Fe I]) are largely insensitive to these effects. With these constraints on the accuracy and precision of high resolution integrated light analyses, detailed abundances are obtained for seven clusters in the outer halo of M31 that were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS) and are likely to have originated in dwarf galaxy satellites. Three clusters are relatively metal rich ([Fe/H] > −1.5) for their locations in the outer halo; their chemical abundances suggest that they likely originated in one or more fairly massive dwarf satellities. The other four are more metal-poor, and may have originated in less massive dwarf satellites. These results indicate that the Milky Way and M31 have both experienced some amount of accretion from dwarf satellites, though M31 may have had a more active accretion history. / Graduate