L’objectif principal de cette thèse est d’identifier les étoiles de faible masse et naines brunes membres d’associations cinématiques jeunes du voisinage solaire. Ces associations sont typiquement âgées de moins de 200 millions d’années et regroupent chacune un ensemble d’étoiles s’étant formées au même moment et dans un même environnement. La majorité de leurs membres d'environ plus de 0.3 fois la masse du Soleil sont déjà connus, cependant les membres moins massifs (et moins brillants) nous échappent encore. Leur identification permettra de lever le voile sur plusieurs questions fondamentales en astrophysique. En particulier, le fait de cibler des objets jeunes, encore chauds et lumineux par leur formation récente, permettra d’atteindre un régime de masses encore peu exploré, jusqu'à seulement quelques fois la masse de Jupiter. Elles nous permettront entre autres de contraindre la fonction de masse initiale et d'explorer la connection entre naines brunes et exoplanètes, étant donné que les moins massives des naines brunes jeunes auront des propriétés physiques très semblables aux exoplanètes géantes gazeuses. Pour mener à bien ce projet, nous avons adapté l'outil statistique BANYAN I pour qu'il soit applicable aux objets de très faibles masses en plus de lui apporter plusieurs améliorations. Nous avons entre autres inclus l'utilisation de deux diagrammes couleur-magnitude permettant de différencier les étoiles de faible masse et naines brunes jeunes à celles plus vieilles, ajouté l'utilisation de probabilités a priori pour rendre les résultats plus réalistes, adapté les modèles spatiaux et cinématiques des associations jeunes en utilisant des ellipsoïdes gaussiennes tridimensionnelles dont l'alignement des axes est libre, effectué une analyse Monte Carlo pour caractériser le taux de faux-positifs et faux-négatifs, puis revu la structure du code informatique pour le rendre plus efficace. Dans un premier temps, nous avons utilisé ce nouvel algorithme, BANYAN II, pour identifier 25 nouvelles candidates membres d'associations jeunes parmi un échantillon de 158 étoiles de faible masse (de types spectraux > M4) et naines brunes jeunes déjà connues. Nous avons ensuite effectué la corrélation croisée de deux catalogues couvrant tout le ciel en lumière proche-infrarouge et contenant ~ 500 millions d’objets célestes pour identifier environ 100 000 candidates naines brunes et étoiles de faible masse du voisinage solaire. À l'aide de l'outil BANYAN II, nous avons alors identifié quelques centaines d'objets appartenant fort probablement à une association jeune parmi cet échantillon et effectué un suivi spectroscopique en lumière proche-infrarouge pour les caractériser. Les travaux présentés ici ont mené à l'identification de 79 candidates naines brunes jeunes ainsi que 150 candidates étoiles de faible masse jeunes, puis un suivi spectroscopique nous a permis de confirmer le jeune âge de 49 de ces naines brunes et 62 de ces étoiles de faible masse. Nous avons ainsi approximativement doublé le nombre de naines brunes jeunes connues, ce qui a ouvert la porte à une caractérisation statistique de leur population. Ces nouvelles naines brunes jeunes représentent un laboratoire idéal pour mieux comprendre l'atmosphère des exoplanètes géantes gazeuses. Nous avons identifié les premiers signes d’une remontée dans la fonction de masse initiale des naines brunes aux très faibles masses dans l'association jeune Tucana-Horologium, ce qui pourrait indiquer que l’éjection d’exoplanètes joue un rôle important dans la composition de leur population. Les résultats du suivi spectroscopique nous ont permis de construire une séquence empirique complète pour les types spectraux M5-L5 à l'âge du champ, à faible (β) et très faible (γ) gravité de surface. Nous avons effectué une comparaison de ces données aux modèles d'évolution et d'atmosphère, puis nous avons construit un ensemble de séquences empiriques de couleur-magnitude et types spectraux-magnitude pour les naines brunes jeunes. Finalement, nous avons découvert deux nouvelles exoplanètes par un suivi en imagerie directe des étoiles jeunes de faible masse identifiées dans ce projet. La future mission GAIA et le suivi spectroscopique complet des candidates présentées dans cette thèse permettront de confirmer leur appartenance aux associations jeunes et de contraindre la fonction de masse initiale dans le régime sous-stellaire. / The main objective of this thesis is the identification of low-mass star and brown dwarf members of young moving groups in the solar neighborhood. These associations are typically younger than 200 million years and include stars formed at the same time and in the same environment. The majority of their members with masses approximately larger than 0.3 times that of the Sun have already been discovered, however the less massive, fainter members are still elusive. Their identification will allow us to address several fundamental questions in astrophysics. In particular, uncovering young objects that are still warm because of their recent formation will allow us to probe masses down to only a few times the mass of Jupiter, a mass regime which is still poorly understood. They will allow us to constrain the initial mass function and explore the connection between brown dwarfs and exoplanets, given that the least massive brown dwarfs have physical properties similar to those of gaseous giant exoplanets. In order to carry through this project, we have adapted the BANYAN I statistical tool to make it applicable to very low-mass objects in addition to bringing several improvements to the tool. We have included the use of two near-infrared color-magnitude diagrams that allow differentiating young low-mass stars and brown dwarfs from older objects, we added the use of prior probabilities to make its results more realistic, we adapted spatial and kinematic models of moving groups using tridimensional gaussian ellipsoids with axes free to rotate, we performed a Monte Carlo analysis to characterize the rate of false-positive and false-negatives, and we revised the structure of its source code to make it more efficient. As a first step, we have used this new algorithm, BANYAN II, to identify 25 new candidate members among a sample of 158 known young low-mass stars (with spectral types > M4) and brown dwarfs. We have then performed a cross-correlation of two all-sky near-infrared catalogs consisting of ~ 500 million celestial objects to identify approximately 100 000 brown dwarf and low-mass star candidates in the solar neighborhood. We have identified a few hundred promising young association members in this sample with the BANYAN II tool, and have performed a near-infrared spectroscopic survey to characterize them. The work presented here has led to the identification of 79 candidate young brown dwarfs and 150 candidate young low-mass stars, and a spectroscopic follow-up allowed us to confirm the young age of 49 brown dwarfs and 62 low-mass stars. We have thus boosted the number of known young brown dwarfs by a factor ~ 2, opening the door to a statistical characterization of their population. These new young brown dwarfs represent an ideal laboratory to better understand the atmospheres of gaseous giant exoplanets. We have identified the first signs of a turn-up in the initial mass function of very low-mass brown dwarfs in the Tucana-Horologium association, which could indicate that exoplanet scattering plays a significant role in composing their population. Results from this spectroscopic follow-up has allowed us to construct an complete empirical sequence of spectral types M5-L5 for field dwarfs, low-gravity (β) and very low-gravity (γ) dwarfs. We have performed a comparison of these new data with evolution and atmosphere models, and constructed a set of empirical spectral type-magnitude and color-magnitude sequences for young brown dwarfs. Finally, we have discovered two new exoplanets from a direct-imaging follow-up of low-mass stars discovered as part of this project. The future GAIA mission and the complete spectroscopic follow-up of the candidates presented in this thesis will allow to confirm their membership and to constrain the initial mass function in the substellar regime.

As distrofias musculares (DM) se caracterizam por degeneração progressiva e irreversível da musculatura esquelética. Dentre estas, a Distrofia Muscular de Duchenne (DMD), constitui doença letal ligada ao cromossomo X, caracterizada pela ausência de distrofina na membrana das fibras musculares, afetando um em cada 3.000 meninos nascidos vivos, sendo que a maioria será confinada à cadeira de rodas a partir dos 6 anos de idade e com sobrevida, sem cuidados especiais, em torno dos 25. Apesar do uso de técnicas modernas de diagnóstico, ainda não existe tratamento eficaz para essas doenças. Alguns trabalhos das décadas de 70 e 80 sugeriram que a inibição do hormônio do crescimento (GH) poderia retardar a evolução do processo distrófico de pacientes com DMD. Entretanto, como essas observações não foram comprovadas cientificamente até a presente data, neste trabalho estudamos os efeitos do excesso e da inibição do GH na evolução do quadro distrófico em modelo murino de distrofia muscular: i) administrou-se GH (10μg/ camundongo/dia, i.p., por 10 dias) em camundongos: a) distróficos dy/dy (B6.Wk-Lama2dy-2J/J) com fenótipo, resultando na aceleração dos sintomas da doença, traduzida pela piora das performances de 17% no teste de deambulação (TD), 64% no teste de sustentação (TS), 28% no teste da esteira (TE) e 55% no teste de força máxima (TFM), em relação aos distróficos que não tomaram GH; apenas no teste de resistência à inclinação (TRI) não houve diferença significativa; entretanto, cortes histológicos do músculo gastrocnêmico desses camundongos, não mostraram alterações significativas em relação aos distróficos que não tomaram a droga; quanto aos marcadores de degeneração e regeneração (MDR) na musculatura desses animais, o GH não alterou os níveis de Pax7, MyoD, Laminina e Desmina, mas aumentou de 90% os níveis de TGFβ-1, uma citocina inflamatória que induz fibrose; b) já nos camundongos normais, o GH, ao contrário dos resultados acima, aumentou as performances de 13% no TD e de 28% TS, e ligeiramente, de forma não significativa, em outros dois testes; cortes histológicos revelaram que o GH aumentou de 16,5% a área de secção transversal dos músculos desses animais, o que poderia explicar os resultados descritos acima; já os níveis dos MDR, incluindo o de TGFβ-1, estavam semelhantes aos dos animais controles; ii) desenvolveu-se um novo modelo experimental resultante de cruzamentos de camundongos geneticamente deficientes em GH (Ghrhrlit) com os distróficos dy/dy (B6.Wk-Lama2dy-2J/J), mostrando um retardo na evolução da doença: as performances dos anões distróficos em relação aos anões normais foram melhores que aquelas dos distróficos em relação aos animais normais, ou sejam, 14% maior TD, 71% maior no TS, 18% maior no TE, 5,5% maior no TRI e 102% maior no TFM; cortes histológicos mostraram que a área do interstício nos músculos dos anões distróficos foi 29% menor que a dos distróficos não anões; os níveis de TGFβ-1 nestes animais, foram os únicos entre aqueles dos MDR estudados, que encontravam-se diminuidos de 36% em relação àqueles dos distróficos não anões; iii) a inibição da liberação do GH pela octreatida, análogo da somatostatina, mostrou também uma relativa melhora ou uma estabilização na evolução do quadro distrófico, traduzido pelo aumento, após 60 dias de tratamento, das performances nos testes funcionais da musculatura em relação àquelas dos animais distróficos sem tratamento: foi 12% maior no TD enquanto os distróficos controles diminuíram em 14%; foi 27% maior no teste de Rota Rod, enquanto os distróficos controles diminuíram em 32%; mantiveram a mesma performance no TE, enquanto os distróficos controles pioraram em 23%, e foram 29% maiores no TS, e 43% no teste de força de preensão, enquanto os distróficos controles não melhoram as suas performances nestes testes. Esses resultados sugerem um efeito patológico do GH na musculatura distrófica, com seu excesso levando a uma piora da doença, possivelmente através do aumento dos níveis de TGFβ-1. A diminuição do GH, consequentemente, poderia levar a um retardo da evolução da doença. Estes estudos experimentais abrem perspectivas bastante promissoras para se testar inibição dos efeitos do GH na evolução das doenças distróficas humanas. / Muscular dystrophies (DM) are characterized by progressive and irrevertible degeneration of the skeleton muscles. Among them, Duchenne Muscular Dystrophy (DMD) consitutes lethal disease linked to chromossome X, distinguished by the absence of dystrophyn in the muscular fibers membrane, affecting one in each 3,000 boys born alive, who, most of them, will be confined to wheel chairs since the age of 6 and with life expectance, without special care, of 25 years. Despite modern diagnosis techniques, there is not an efficient treatment for these diseases. Some works from the 1970´s and 1980´s suggested that the inhibition of the growth hormone (GH) could retard the dystrophy process evolution in DMD patients. Nevertheless since this observation has not been proven scientifically, in the present work, the GH excess and inhibition effects on a murino model of muscular dystrophy, were studied: i) GH (10μg/mouse/day, i.p., for 10 days) was given to mice: a) dystrophic dy/dy (B6.Wk-Lama2dy-2J/J), with phenotype , resulting in the acceleration of the disease symptoms, confirmed by the performance worsening by 17% in the deambulation test (DT), 64% in the sustaining test (S), 28% in the mat test and 55% in the maximum strength test (MST), in relation to dystrophic mice which had not taken GH; only in the inclination resistance test (IRT) there was no significant difference; however, histological cuts of the gastrocnemius muscle of these mice did not show significant alterations, comparing to dystrophic mice which had not taken the drug: as to degeneration and regeneration markers (DRM) in these animals muscles, the GH did not alter Pax7, MyoD. Laminin and Desmin levels, although it increased by 90% the TGFβ-1 levels, an inflammatory cytokine, that induces fibrosis b) in normal mice, the GH, opposite to the results above, increased the performances by 13% in the DT and by 28% in the ST, and slightly, not significantly, in other two tests; histological cuts revealed that the GH increased by 16.5% the transversal section area of these animals muscles, what could explain the results described above; as to the MDR levels, including those of the TGFβ-1, they were similar to the control animals; ii) a new experimental model, resulting from the crossing of GH genetically deficient mice (Ghrthlit) with dystrophic dy/dy (B6.Wk-Lama2dy-2J/J), showing retardation in the disease evolution; the performances of the dystrophic dwarfs, compared to the normal dwarfs, were better than those of the dystrophic compared to normal animals, namely, 14% larger DT, 71% superior in the ST, 18% larger in the MT, 5.5% larger in the IRT and 102% superior in the MST; histological cuts showed that the interstice area in the dystrophic dwarfs muscles was 29% smaller than that of non-dwarfs dystrophic; the TGFβ-1 levels in these animals were the only MDR studied, which were found to be diminished by 36%, compared to those of the non-dwarfs dystrophic; the GH release inhibition by octreotide, somatostatin analogous, also showed a relative improvement or stabilization in the dystrophy evolution, demonstrated by the increase, ater 60 days treatment, of the performances in the muscle functional tests, compared to those of the dystrophic animals, without treatment: it was 12% larger in the DT, while the control dystrophic animals worsened by 32%; they maintained the same performance in the ST, and 43% in the seizing strength test, while the control dystrophic did not improve their performances in these tests. These results suggest a GH pathologic effect, possibly through the TGFβ-1 levels increase. The GH diminishing, consequently, could lead to the retardation in the disease evolution. These experimental studies open promissing perspectives to test the GH effects inhibition in human dystrophic diseases evolution.

Les naines brunes sont des objets de masse intermédiaire entre celle nécessaire pour former une étoile et celle d'une planète. Les naines brunes sont classées, des plus chaudes aux plus froides, en types spectraux L, T et Y, caractérisés par une couleur J-K moyenne qui varie de 1.2 à 1.8 pour les étoiles de type L0 à L8, et de 1.8 à -0.5 pour les étoiles de type L8 à T8. Par ailleurs, la couleur J-K de certains types spectraux présente une dispersion de l'ordre d'une magnitude. Ce travail tente de faire la lumière sur la nature de cette grande dispersion, présente dans la couleur J-K des naines brunes de type L2. Les observations ont été réalisées avec la caméra infrarouge CPAPIR à l'Observatoire du Mont Mégantic. Nous avons ciblé un total de 22 naines brunes qui ont été observées en K, et 12 parmi celles-ci ont aussi été observées en J. Chacune des naines brunes a été calibrée à l'aide d'une étoile standard, ce qui rend nos résultats indépendants des données 2MASS. Nous observons une corrélation entre les couleurs J-K de nos données et de celles de 2MASS. Cela montre que la grande dispersion en J-K de nos données et des données 2MASS est due aux propriétés physiques des naines brunes et non à des erreurs observationnelles. L'examen des facteurs qui pourraient être responsables de cette grande dispersion, soit la classification spectrale, la métallicité, la gravité de surface, une binarité non résolue, la présence de nuages de condensats et la rotation, montre que la gravité de surface serait le facteur le plus susceptible d'être responsable de la grande dispersion des valeurs de J-K. / Brown dwarfs are objects with a mass intermediate between that required to form a star and that of a planet. Brown dwarfs are classified, from higher to lower temperature, under spectral types L, T and Y, caracterized by a J-K average color that varies from 1.2 to 1.8 for types L0 to L8, and from 1.8 to -0.5 for types L8 to T8. Furthermore, the J-K color of some spectral types presents more than a magnitude of dispersion. This study attempts to explain the large dispersion of the J-K color of the type L2 brown dwarfs. Observations were made with the infrared camera CPAPIR at the Observatoire du Mont Mégantic. We targeted a total of 22 brown dwarfs that were observed in the K band, and 12 among them were also observed in the J band. Each brown dwarf was calibrated with a standard star, which makes our data independent from those of 2MASS. We observe a correlation between the J-K colors obtained from our data and those from 2MASS. This shows that the large J-K dispersion in the data is due to brown dwarf physical properties and not to observational errors. Consideration of the factors that could be responsible for this large dispersion, namely the spectral classification, the metallicity, the surface gravity, an unresolved binarity, the presence of clouds and rotation, shows that surface gravity is the factor most likely to explain the large J-K color dispersion.

Depuis la première détection d'une planète extrasolaire autour d'une étoile de type solaire par Mayor et Queloz (1995), plus de 1500 planètes ont été découverts. Actuellement il existe un énorme intérêt à découvrir et caractériser des planètes semblables à la Terre, en particulier celles situées dans la zone habitable de leur étoile hôte (définie comme la distance à l'étoile hôte où la température de la planète permet l'existence d'eau liquide à la surface). La détection de planètes de type terrestre, et la recherche de biomarqueurs dans leurs atmosphères sont parmi les principaux objectifs de l'astronomie du vingt et unième siècle. La méthode des vitesses radiales (VR), consistant à mesurer le mouvement réflexe de l'étoile induit par des planètes en orbite, est une remarquable technique pour atteindre cet objectif.Pour atteindre les précisions nécessaire à la detection de telles planètes il est absolument nécessaire de concevoir des spectrographes extrêmement stables, d'avoir une très bonne compréhension de l'activité stellaire (qui peut mimer l'effet d'une planète), d'effectuer un traitement soigneux de l'atmosphère terrestre (laquelle inévitablement laisse des empreintes dans les spectres acquis depuis le sol), et de disposer d'une puissante technique pour extraire, à partir des spectres, autant d'information Doppler que possible. La recherche de planètes orbitant autour des étoiles de très faible masse, plutôt qu'autour des étoiles de type solaire, permet d'aborder dès maintenant la détection de planètes de faible masse dans la zone habitable. En effet, en gardant tout les autres paramètres égaux, le mouvement réflexe (et donc l'amplitude de la variation VR) sera plus grande si l'étoile centrale est de très faible masse. De plus les naines M ont une plus faible luminosité que les étoiles de type solaire, il en resulte des périodes orbitales courtes des planètes dans la zone habitable (~50 jours pour les naines M contre ~360 jours pour des étoiles de type solaire), entraînant à nouveau en une plus grande amplitude des VR. Une précision de ~1 m/s en VR permet la détection d'une planète dans la zone habitable d'une naine M, alors que ~0.1 m/s sont nécessaire dans le cas d'une étoile de type solaire.Cette thèse vise à optimiser l'extraction de VR des spectres des naines M à haute résolution acquis avec le spectrographe HARPS (avec une possibilité d'applications futures sur d'autres instruments comme SOPHIE, HARPS-N et le prochain spectrographe infrarouge SPIRou - prochainement mis en service au CFHT). Les effets de l'activité stellaire des naines M seront également analysées, dans le contexte de la technique des VR. Divers traceurs d'activité stellaire sont utilisés pour rejeter des fausses détections ou pour étudier les relations entre l'activité magnétique et la rotation. Dans cette thèse (Chap. 3) je calibre pour la première fois le flux dans les raies H et K du Calcium en fonction de la luminosité bolométrique et je détermine la relation entre cet estimateur R'HK et la période de rotation des naines M. Dans le chapitre 4 je décris l'implémentation d'une méthode d'extraction de VR par une minimisation du Chi-deux entre un template spectral et les spectres observés. Je démontre que cette méthode est plus précise que celle classiquement utilisée. Les raies telluriques qui affectent les mesures VR sont prises en compte dans les procédures d'analyse. Ces méthodes sont testées sur des systèmes avec des candidats planétaires, je discuterais l'analyse de certains de ces systèmes. / Since the first detection of an extrasolar planet orbiting a Sun-like star by Mayor and Queloz (1995), more than 1500 have been discovered. Enormous interest is currently focused on finding and characterising Earth-like planets, in particular those located in the habitable zone of their host star (defined as the distance from the host star where the planet temperature allows liquid water to flow on its surface). Both the detection of Earth-like planets, and the search for biomarkers in their atmospheres are among the main objectives of the twenty-first century's astronomy. The method known as radial velocities (RV), that consists in the measure of the star's reflex motion induced by orbiting planets, is a promising technique to achieve that quest.The main difficulties with the RV technique are the needs of an extremely stable spectrograph, a correct understanding of stellar activity (which can mimic the effect of a planet), a careful treatment of our Earth's atmosphere (which inevitable imprints spectra taken from the ground), and the need to dispose of a powerful algorithm to extract as much Doppler information as possible from the recorded spectra. Search for planets orbiting very low-mass stars (M dwarfs) can more easily reach the goal of detecting low-mass planets in the habitable zone of their parent star, compared to solar-type stars. Indeed, everything else being equal, a lower mass of the host star implies a larger reflex motion, and thus a larger RV amplitude. Moreover, the lower luminosity of M dwarfs compared to Sun-like stars, implies shorter orbital periods from planets in the habitable zone (~50 days against ~360 days, for M dwarfs compared to solar-type stars, respectively), resulting again in a larger RV amplitude. A RV precision of ~1 m/s allows a planet detection in the habitable zone of an M dwarf, whereas ~0.1 m/s is required in the case of a solar-type stars.This thesis aims to optimise the RV extraction from HARPS high-resolution spectra (and to open similar analysis on other instruments like SOPHIE, HARPS-N and the upcoming infrared spectrograph SPIRou -- to be commissioned to the 3.6-m CFH-Telescope). The effects of stellar activity will also be analysed, and contextualised in the RV technique. Stellar activity tracers are used to reject false detections or to study the relationships between the stellar magnetic activity and rotation. In this thesis (Chap.ref{chap:mag_activity}) I calibrate for the first time the ratio between the Ca textrm{small II} Htextrm{small &}K chromospheric lines and the bolometric luminosity for M dwarfs. I determine a relationship between the R^prime_{HK}-index and the rotation period of M dwarfs. In chapter~ref{chap:template_matching} I describe my algorithm to extract RVs through a chi^2-minimisation between a stellar template and the observed spectra. I demonstrate the improved accuracy of this method. Telluric spectral lines also affect the measurements of RV and are taken into account in the analysis procedures. I tested these methods on systems with planetary candidates, and for some systems, I took in charge the Keplerian analysis.

J'ai consacré mon travail de thèse à la recherche de quasars à haut redshift dans le sondage Canada France High-z Quasar Survey in the Near Infrared (CFHQSIR). L'objectif principal de ce programme est de découvrir des quasars de redshift de l'ordre de z $\sim$ 7 dans les champs larges du CFHTLS (CFHTLS Wide) en utilisant des images réalisées dans le proche infrarouge avec la caméra WIRCam installée au foyer du CFHT. J'ai tout d'abord effectué la calibration photométrique de l'ensemble des données de CFHQSIR. J'ai ensuite étudié la qualité des images ainsi que leur propriété de bruit et leur profondeur. J'ai dans un deuxième temps calculé le nombre de quasars qu'il serait possible de détecter avec CFHQSIR et en ai déduit les contraintes envisageables sur la fonction de luminosité des quasars à z $\sim$ 7. J'ai ensuite procédé à l'identification de candidats quasars parmi les dizaines de milliers de sources que comptent les 130 degrés carrés couvert par CFHQSIR. Des observations de suivi photométriques des candidats ont finalement permis de révéler une cinquantaine d’objets, dont une quinzaine a été sélectionnée pour des observations spectroscopiques au Very Large Telescope (VLT). Celles-ci n’ayant, pour la plupart, pas encore été realisées, la nature exacte de ces sources ne pourra être connue que dans les prochains mois. Pour finir, la dernière partie de ma thèse s’est focalisée sur l’étude de modèles statistiques bayésiens afin de compléter ma méthode de sélection de candidats quasars. Cette étude m'a finalement permis de vérifier que la majorité des objets retenus pour des observations spectroscopiques étaient effectivement les candidats les plus probables. / My PhD work is focused on the search for high-redshift quasars in the Canada France High-z Quasar Survey in the Near Infrared (CFHQSIR). The main scientific objective of this CFHT Large Program is to search for quasars at redshift z $\sim$ 7 with near-infrared images of the CFHTLS Wide fields acquired with the CFHT WIRCam camera. Firstly, I carried out the photometric calibration of the CFHQSIR images. I performed a detailed analysis of the CFHQSIR data by studying their quality, as well as their noise properties and their depths.Secondly, I computed the number of high-redshift quasars that could be detected with CFHQSIR and the corresponding constraints which could be put on the z $\sim$ 7 quasar luminosity function. Then, I proceeded to the identification of quasar candidates among the many thousands of sources in the 130 square degrees covered by CFHQSIR. Photometric follow-up observations of the candidates revealed about fifty objects, of which fifteen were chosen to be spectroscopically observed with the Very Large Telescope (VLT). Given that this spectroscopic follow-up is not yet completed, the exact nature of these sources will only be known in the coming months. To finish, I applied Bayesian model comparison to my sample in order to complete and consolidate my selection procedure. My candidates were finally classified according to their probability to be a high-redshift quasar. I verified that the majority of the most likely candidates were selected for spectroscopic observations.

As distrofias musculares (DM) se caracterizam por degeneração progressiva e irreversível da musculatura esquelética. Dentre estas, a Distrofia Muscular de Duchenne (DMD), constitui doença letal ligada ao cromossomo X, caracterizada pela ausência de distrofina na membrana das fibras musculares, afetando um em cada 3.000 meninos nascidos vivos, sendo que a maioria será confinada à cadeira de rodas a partir dos 6 anos de idade e com sobrevida, sem cuidados especiais, em torno dos 25. Apesar do uso de técnicas modernas de diagnóstico, ainda não existe tratamento eficaz para essas doenças. Alguns trabalhos das décadas de 70 e 80 sugeriram que a inibição do hormônio do crescimento (GH) poderia retardar a evolução do processo distrófico de pacientes com DMD. Entretanto, como essas observações não foram comprovadas cientificamente até a presente data, neste trabalho estudamos os efeitos do excesso e da inibição do GH na evolução do quadro distrófico em modelo murino de distrofia muscular: i) administrou-se GH (10μg/ camundongo/dia, i.p., por 10 dias) em camundongos: a) distróficos dy/dy (B6.Wk-Lama2dy-2J/J) com fenótipo, resultando na aceleração dos sintomas da doença, traduzida pela piora das performances de 17% no teste de deambulação (TD), 64% no teste de sustentação (TS), 28% no teste da esteira (TE) e 55% no teste de força máxima (TFM), em relação aos distróficos que não tomaram GH; apenas no teste de resistência à inclinação (TRI) não houve diferença significativa; entretanto, cortes histológicos do músculo gastrocnêmico desses camundongos, não mostraram alterações significativas em relação aos distróficos que não tomaram a droga; quanto aos marcadores de degeneração e regeneração (MDR) na musculatura desses animais, o GH não alterou os níveis de Pax7, MyoD, Laminina e Desmina, mas aumentou de 90% os níveis de TGFβ-1, uma citocina inflamatória que induz fibrose; b) já nos camundongos normais, o GH, ao contrário dos resultados acima, aumentou as performances de 13% no TD e de 28% TS, e ligeiramente, de forma não significativa, em outros dois testes; cortes histológicos revelaram que o GH aumentou de 16,5% a área de secção transversal dos músculos desses animais, o que poderia explicar os resultados descritos acima; já os níveis dos MDR, incluindo o de TGFβ-1, estavam semelhantes aos dos animais controles; ii) desenvolveu-se um novo modelo experimental resultante de cruzamentos de camundongos geneticamente deficientes em GH (Ghrhrlit) com os distróficos dy/dy (B6.Wk-Lama2dy-2J/J), mostrando um retardo na evolução da doença: as performances dos anões distróficos em relação aos anões normais foram melhores que aquelas dos distróficos em relação aos animais normais, ou sejam, 14% maior TD, 71% maior no TS, 18% maior no TE, 5,5% maior no TRI e 102% maior no TFM; cortes histológicos mostraram que a área do interstício nos músculos dos anões distróficos foi 29% menor que a dos distróficos não anões; os níveis de TGFβ-1 nestes animais, foram os únicos entre aqueles dos MDR estudados, que encontravam-se diminuidos de 36% em relação àqueles dos distróficos não anões; iii) a inibição da liberação do GH pela octreatida, análogo da somatostatina, mostrou também uma relativa melhora ou uma estabilização na evolução do quadro distrófico, traduzido pelo aumento, após 60 dias de tratamento, das performances nos testes funcionais da musculatura em relação àquelas dos animais distróficos sem tratamento: foi 12% maior no TD enquanto os distróficos controles diminuíram em 14%; foi 27% maior no teste de Rota Rod, enquanto os distróficos controles diminuíram em 32%; mantiveram a mesma performance no TE, enquanto os distróficos controles pioraram em 23%, e foram 29% maiores no TS, e 43% no teste de força de preensão, enquanto os distróficos controles não melhoram as suas performances nestes testes. Esses resultados sugerem um efeito patológico do GH na musculatura distrófica, com seu excesso levando a uma piora da doença, possivelmente através do aumento dos níveis de TGFβ-1. A diminuição do GH, consequentemente, poderia levar a um retardo da evolução da doença. Estes estudos experimentais abrem perspectivas bastante promissoras para se testar inibição dos efeitos do GH na evolução das doenças distróficas humanas. / Muscular dystrophies (DM) are characterized by progressive and irrevertible degeneration of the skeleton muscles. Among them, Duchenne Muscular Dystrophy (DMD) consitutes lethal disease linked to chromossome X, distinguished by the absence of dystrophyn in the muscular fibers membrane, affecting one in each 3,000 boys born alive, who, most of them, will be confined to wheel chairs since the age of 6 and with life expectance, without special care, of 25 years. Despite modern diagnosis techniques, there is not an efficient treatment for these diseases. Some works from the 1970´s and 1980´s suggested that the inhibition of the growth hormone (GH) could retard the dystrophy process evolution in DMD patients. Nevertheless since this observation has not been proven scientifically, in the present work, the GH excess and inhibition effects on a murino model of muscular dystrophy, were studied: i) GH (10μg/mouse/day, i.p., for 10 days) was given to mice: a) dystrophic dy/dy (B6.Wk-Lama2dy-2J/J), with phenotype , resulting in the acceleration of the disease symptoms, confirmed by the performance worsening by 17% in the deambulation test (DT), 64% in the sustaining test (S), 28% in the mat test and 55% in the maximum strength test (MST), in relation to dystrophic mice which had not taken GH; only in the inclination resistance test (IRT) there was no significant difference; however, histological cuts of the gastrocnemius muscle of these mice did not show significant alterations, comparing to dystrophic mice which had not taken the drug: as to degeneration and regeneration markers (DRM) in these animals muscles, the GH did not alter Pax7, MyoD. Laminin and Desmin levels, although it increased by 90% the TGFβ-1 levels, an inflammatory cytokine, that induces fibrosis b) in normal mice, the GH, opposite to the results above, increased the performances by 13% in the DT and by 28% in the ST, and slightly, not significantly, in other two tests; histological cuts revealed that the GH increased by 16.5% the transversal section area of these animals muscles, what could explain the results described above; as to the MDR levels, including those of the TGFβ-1, they were similar to the control animals; ii) a new experimental model, resulting from the crossing of GH genetically deficient mice (Ghrthlit) with dystrophic dy/dy (B6.Wk-Lama2dy-2J/J), showing retardation in the disease evolution; the performances of the dystrophic dwarfs, compared to the normal dwarfs, were better than those of the dystrophic compared to normal animals, namely, 14% larger DT, 71% superior in the ST, 18% larger in the MT, 5.5% larger in the IRT and 102% superior in the MST; histological cuts showed that the interstice area in the dystrophic dwarfs muscles was 29% smaller than that of non-dwarfs dystrophic; the TGFβ-1 levels in these animals were the only MDR studied, which were found to be diminished by 36%, compared to those of the non-dwarfs dystrophic; the GH release inhibition by octreotide, somatostatin analogous, also showed a relative improvement or stabilization in the dystrophy evolution, demonstrated by the increase, ater 60 days treatment, of the performances in the muscle functional tests, compared to those of the dystrophic animals, without treatment: it was 12% larger in the DT, while the control dystrophic animals worsened by 32%; they maintained the same performance in the ST, and 43% in the seizing strength test, while the control dystrophic did not improve their performances in these tests. These results suggest a GH pathologic effect, possibly through the TGFβ-1 levels increase. The GH diminishing, consequently, could lead to the retardation in the disease evolution. These experimental studies open promissing perspectives to test the GH effects inhibition in human dystrophic diseases evolution.

Ce mémoire présente, pour la première fois, des analyses détaillées d’étoiles naines blanches montrant à la fois des éléments lourds et de forts champs magnétiques. Ces analyses sont effectuées à l’aide d’un tout nouveau code d’atmosphère pouvant calculer la position et la force des raies de tous les éléments (du carbone au cuivre) en régime Paschen-Back pour une géométrie de notre choix. Dans un premier temps, nous décrivons l'effet d'un champ magnétique sur les niveaux d'énergie atomiques, par l'effet Zeeman en champ faible puis en régime Pashen-Back lorsque l'approche perturbative n'est plus valide. Nous explorons ensuite l’effet de la géométrie du champ magnétique, notamment les différences entre les modèles calculés en supposant un champ magnétique uniforme (ou constant dans l'espace) et ceux ayant une géométrie dipolaire. Certaines études ont récemment affirmé qu’en présence d’un champ magnétique intense le mouvement convectif était fortement atténué, de sorte que l’atmosphère devenait pratiquement radiative (Tremblay et al., 2015). Nous explorons brièvement l’impact que peut avoir la suppression du transport d’énergie convective sur les résultats d’analyses d’étoiles magnétiques. Dans le but d'améliorer les analyses d'étoiles magnétiques, nous avons implémentés certains changements aux modèles d'atmosphères utilisés, tel que l'ajout du traitement magnétique des raies métalliques. Nos nouveaux outils nous permermetterons d'analyser de manière rigoureuse, pour la première fois, les étoiles G165-7, J2105+0900 et LHS 2534, trois étoiles magnétiques avec présence d'éléments lourds. Nous étudions leurs paramètres atmosphériques tels que la température effective, gravité de surface et abondances de métaux à partir de modèles standards ainsi qu’à partir de modèles radiatifs ou la convection a été artificiellement inhibée par la présence d’un champ magnétique intense. Nous observons que nos modèles riches en hydrogène sans convection ne reproduisent plus les observations, contrairement à ceux riches en hélium où la convection se produit hors de la région de formation des raies (soit plus profondément). Nous observons finalement qu'un champ magnétique dipolaire centré avec l'étoile ne reproduit pas bien les profils des raies d'un même multiplet, puisque les composantes sigma (celles à gauche et à droite de la raie centrale) prédites sont trop larges. Les observations spectroscopiques sont mieux reproduites à partir de modèles ayant un champ magnétique de surface constant, ou avec un dipôle toujours fortement décalé selon l’axe -z, indiquant que la géométrie réelle du champ magnétique est probablement plus complexe qu’un simple dipôle. / We present, for the first time, detailed analyses of white dwarf stars showing heavy metal lines and large magnetic fields. These analyses are carried out with a new atmosphere code able to compute the position and strength of lines for all elements (from carbon to copper) in the Paschen-Back regime for an arbitrary geometry. Firstly, we describe the effect of a magnetic field on the atomic energy levels, first with the Zeeman effect for weak fields then in the Paschen-Back regime when the pertubative approach is no longer valid. We then explore the effect of the magnetic field geometry, especially the differences between models calculated assuming a constant field and those with a dipolar geometry. Some studies recently suggested that in the presence of an intense magnetic field, the convective movement is strongly inhibited, renderingthe atmosphere mostly radiative (Tremblay et al., 2015). We briefly explore the impact the suppression of the convective energy transport can have on the analyses' results of magnetic stars. In order to carry out a thorough analysis of some magnetic stars, we have applied changes to the atmospheric models used, for example by adding magnetic treatment of metallic lines. With these new tools, we analyse for the first time, in a rigorous manner, the stars G165-7, J2105+0900 and LHS 2534, three magnetic stars showing heavy elements. We study their atmospheric parameters such as the effective temperature, surface gravity and metal abundances, and then explore the possibility of having pure radiative atmospheres, where the convection would be inhibited by intense magnetic fields. We find that hydrogen rich models cannot reproduce observations, unlike those rich in helium where the convective movement takes place out of the line forming region. We finally note that a centered dipolar magnetic field is not able to reproduce the line profiles of a given multiplet, since the sigma components (those to the left and the right of the central line) predicted are too broad. Spectroscopic observations are better reproduced with constant magnetic field models, or with a strongly offset dipole in the -z axis, indicating that the actual field geometry probably is more complex than a simple dipole.

Magister Scientiae - Msc / This thesis reports the results of a twofold study on the recently proposed phenomenon of ‘stellar pulsations’ in young brown dwarfs by the seminal study of Palla and Baraffe (2005) (PB05, thereafter). The PB05 study presents results of a non-adiabatic linear stability analysis showing that young brown dwarfs should become pulsationally unstable during the deuterium burning phase of their evolution. / South Africa

The works deals with the study of the flare stars, which is a group of stars for which sudden brightening can be observed. The work focuses on a star GJ 3236, which is a low-mass eclipsing binary and on which numerous flares have been observed. For the analysis of this system spectroscopic and photometric data were used, which were obtained at various observatories. Parameters of the binary system have been determined by analysing spectroscopic and photometric data with the program PHOEBE. A total of 241 flares have been detected in the photometric data and for 190 flares, which light curves were not affected by eclipses, released energies were estimated. The set of flares was used for the study of stellar activity of the binary. The energy distribution of observed flares is similar to the flares observed on other flare stars and also on the Sun. This suggests, that the flare mechanism is the same for these stars.

Cette thèse présente les avancées dans l'étude des étoiles naines blanches magnétiques. L'étude des étoiles naines blanches en général, le state final d'évolution de près de 95% des étoiles de notre galaxie, permet d'obtenir de l'information cruciale sur leurs progéniteurs. Se pencher sur les propriétés des champs magnétiques des étoiles naines blanches par opposition à celles de leur progéniteur, beaucoup plus difficiles à observer en partie à cause de la faible intensité du champ, est bien plus réalisable. Plusieurs hypothèses ont été formulées afin de décrire l'origine et l'évolution de ces objets atypiques. Que les champs soient d'origine fossile suite à l'évolution d'une étoile isolée et conservation du flux magnétique, ou créés lors de la rotation rapide de la fusion de deux étoiles, il nous faut bien comprendre leur résultat, les naines blanches magnétiques, avant de pouvoir retracer leur évolution. Toutefois, les modèles de naines blanches magnétiques d'aujourd'hui reposent sur un nombre d'approximation, qui ne sont valides jusqu'à une relativement faible intensité de champ. Ceci a été la motivation derrière ce projet de doctorat. Nous avons implémenté un traitement des raies d'absorption utilisant le calcul des énergies de transition de l'atome d'hydrogène et d'hélium sous champs magnétiques arbitraires. Ces calculs, fournis par le groupe de l'université de Tübingen en Allemagne, ont utilisé une approche numérique à la résolution de l'Hamiltonien de l'atome d'hydrogène et d'hélium en présence d'un champ magnétique d'intensité arbitraire. Cet ajout aux modèles d'atmosphères permet de modéliser la position et l'intensité des raies d'absorption dans le spectre des étoiles. Une procédure de discrétisation de la surface visible de l'étoile nous permet de modéliser des champs magnétiques avec une géométrie arbitraire, tel qu'un dipôle décentré et incliné par rapport à l'observateur. Ces nouveaux outils ont été utilisés afin d'étudier en premier lieu un ensemble des naines blanches magnétiques avec une atmosphère riche en hydrogène. De ces résultats, nous avons déterminé une distribution de masse de ces étoiles, avec une masse moyenne significativement plus élevée que leurs homologues non-magnétiques, tels qu'attendu par les théories d'évolution binaires. Les informations extraites des différentes caractéristiques des distributions obtenues suite à cette analyse ne nous permettent cependant pas de favoriser une hypothèse d'évolution plutôt qu'une autre. Suite à ceci, celles avec une atmosphère riche en hélium ont été les prochaines à être analysées. Nous avons ainsi déterminé rigoureusement, pour la première fois, les paramètres magnétiques de ces étoiles riches en hélium. / This thesis presents advances in the study of magnetic white dwarf stars. The study of white dwarf stars in general, the final state of evolution of nearly 95% of the stars in our galaxy, provides crucial information about their progenitors. It is much more feasible to study the magnetic field properties of white dwarf stars as opposed to their progenitor stars, which are much more difficult to observe in part because of the weak field strength. Several hypotheses have been formulated to describe the origin and evolution of these atypical objects. Whether the fields are of fossil origin following the evolution of an isolated star and conservation of magnetic flux, or created during the rapid rotation of the merger of two stars, we need to understand their result, the magnetic white dwarfs, before we can trace their evolution. However, today's models of magnetic white dwarfs rely on a number of approximations, which are only valid up to relatively low field strengths. This was the motivation behind this doctoral thesis. We have implemented a treatment of the absorption lines using the calculation of the transition energies of the hydrogen and helium atom under arbitrary magnetic fields. These calculations, provided by the group of the University of Tubingen in Germany, used a numerical approach to the resolution of the Hamiltonian of the hydrogen and helium atom in the presence of a magnetic field of arbitrary intensity. This addition to atmospheric models allows to model the position and intensity of absorption lines in the spectrum of stars. A discretization procedure of the visible surface of the star allows us to model magnetic fields with an arbitrary geometry, such as an offset and inclined dipole. These new tools have been used to study first a set of magnetic white dwarfs with a hydrogen-rich atmosphere. From these results, we have determined a mass distribution of these stars, with an average mass significantly higher than their non-magnetic counterparts, as expected by the binary evolution theories. The information extracted from the different characteristics of the distributions obtained from this analysis does not however allow us to favor one evolutionary hypothesis over another. Following this, those with a helium rich atmosphere were the next to be analyzed. We have thus determined rigorously, for the first time, the magnetic parameters of these helium-rich stars.