Modelos computacionais existentes de estrelas quentes com rotação moderada ou rápida (vrot >= 20 km/s) não reproduzem satisfatoriamente as caractersticas das freqüências observadas devido às pulsações não-radiais, como por exemplo splittings e assimetrias. O objetivo do trabalho consiste em melhorar a qualidade de tais modelos de forma a poder cotejá-los com as caractersticas observacionais de pulsações não-radiais num processo iterativo que conduzirá a determinar com maior precisão os parâmetros fsicos e a estrutura interna de tais estrelas. Em particular, procuramos determinar o perfil radial de rotação no interior dos objetos. Os satélites da geração atual (Corot, Kepler, etc.) permitem medir os parâmetros das pulsações não-radiais com grande sensibilidade de detecção e grande poder de resolução de frequências. Apresentamos os resultados de nossos cálculos, cotejados com resultados obtidos a partir de observações fotométricas e espectroscópicas da estrela de tipo beta Cephei theta Ophiuchi (vsini = 29 +/- 7 km/s). Com tal procedimento, conseguimos: (i) identificar as caractersticas do perfil de rotação interna das estrelas quentes, indispensável para fazer modelos mais realistas; e (ii) simplificar o problema: refazemos o código de cálculo perturbativo contendo apenas os termos dominantes quanto à sensibilidade à rotação, tornando mais preciso e eficiente o cálculo de pulsações não-radiais para as estrelas estudadas. / Current computational models for hot stars with moderate or rapid rotation (vrot >= 20 km/s) do not satisfactorily reproduce the characteristics of observed frequencies due to non-radial pulsations, for instance, splittings and asymmetries. The goal of this work is to improve the quality of such models in such a way that they better represent observational characteristics of non-radial pulsations, in an iterative process which leads to better precision of physical parameters and internal structure of such stars. In particular, we determine the radial rotation profile in the interior of the objects. Present-day satellites (Corot, Kepler, etc.) allow measurement of non-radial pulsation parameters with great detection sensibility and high-resolution frequency power. We present the results of our calculations, compared with results obtained from photometric and spectroscopic observations from the beta Cephei star theta Ophiuchi (vsini = 29 +/- 7 km/s). With such a procedure, we were able to: (i) identify the characteristics of internal rotation profiles of hot stars, which are needed for more realistic models; and (ii) simplify the problem: we redid the perturbative calculation code including only the main terms in relation to the sensibility to rotation, yielding a more precise and efficient calculus of non-radial pulsation for the stars studied.

The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.

Photometric techniques are often used to observe stars and it can be demonstrated that fundamental stellar properties can be observationally determined using calibrated sets of photometric data. Many of the most powerful techniques utilized to calibrate stellar photometry employ the use of stars in clusters since the individual stars are believed to have many common properties such as age, composition, and approximate distance. Broadband photometric Johnson/Cousins BVRI observations are presented for several nearby open clusters. The new photometry has been tested for consistency relative to archival work and shown to be both accurate and precise. The careful use of a regular routine when making photometric observations, along with the monitoring of instrumental systems and the use of various quality control techniques when making observations or performing data reductions, will enhance an observer's ability to produce high-quality photometric measurements. This work contains a condensed review of the history of photometry, along with a brief description of several popular photometric systems that are often utilized in the field of stellar astrophysics. Publications written by Taylor or produced during the early Taylor and Joner collaboration are deemed especially relevant to the current work. A synopsis of seven archival publications is offered, along with a review of notable reports of VRI photometric observations for the nearby Hyades open star cluster. The body of this present work consists of four publications that appeared between the years 2005 and 2008, along with a soon to be submitted manuscript for a fifth publication. Each of these papers deals specifically with high-quality broadband photometry of open clusters with new data being presented for the Hyades, Coma, NGC 752, Praesepe, and M67. It is concluded that the VRI photometry produced during the Taylor and Joner collaborative investigations forms a high-quality data set that has been: 1) stable for a period of more than 25 years; 2) monitored and tested several times for consistency relative to the broadband Cousins system, and 3) shown to have well-understood transformations to other versions of broadband photometric systems. Further work is suggested for: 1) the transformation relationships for the reddest stars available for use as standards; 2) the standardization of more fields for use with CCD detectors; 3) a further investigation of transformations of blue color indices for observations done using CCD detectors with enhanced UV sensitivity, and 4) a continuation of work on methods to produce high-quality observations of assorted star clusters (both open and globular) with CCD-based instrumentation and intermediate-band photometric systems.

Les études Galactiques nécessitent de grands échantillons d'étoiles dont la masse, l'âge, les abondances, la vitesse et la distance sont connues. Les observations spectroscopiques permettent de mesurer certains de ces paramètres et les autres sont soit déterminés par d'autres moyens, ou dérivés par le calcul. Le besoin d'échantillons statistiquement représentatifs a motivé la construction d'instruments, et la réalisation de grands relevés comme le SDSS, LAMOST et GAIA... Ces projets génèrent une énorme quantité de données que les méthodes d'analyse interactives traditionnelles ne peuvent pas gérer. Cela a motivé des efforts pour concevoir des méthodes automatiques. Mon travail a commencé dans ce contexte, et les objectifs étaient de développer et tester une méthode automatique, puis de l'appliquer à des spectres stellaires à moyenne résolution. La thèse comporte quatre sections : 1. Je présente le package informatique ULySS, et en particulier son application à la détermination des paramètres atmosphériques des étoiles. 2. Nous avons utilisé ULySS pour déterminer les paramètres atmosphériques des 1273 étoiles de la bibliothèque CFLIB. 3. Nous avons appliqué la même méthode sur des observations obtenus pendant la mise en service de LAMOST et nous avons préparé une base de données de spectres de références pour les relevés futurs avec cet instrument. 4. Nous avons cherché des étoiles présumées pauvres en métaux (MP) en se servant des données de vérification scientifique de LAMOST. Cette étude contribue à l'étude des étoiles MP dans la Voie Lactée et montre la faisabilité de ces recherches avec LAMOST et ULySS / Galactic studies require large samples of stars with known mass, age, abundance, spatial velocity and distance etc. Spectroscopic observations allow ones to measure some of these parameters and to derive the others. The need for statistically representative samples motivated the construction of instruments and the realization of large surveys like SDSS, LAMOST and GAIA... These projects bring an enormous quantity of data that the traditional interactive spectral analysis methods cannot handle. This triggered efforts to design automatic methods. My work started in this context, and the goals were to develop and test an automatic method and to apply it to medium resolution stellar spectra. The thesis contains four sections: 1. The ULySS package, and in particular its application to the determination of the atmospheric parameters of stars is presented. 2. We used ULySS to determine the atmospheric parameters of the 1273 stars of the CFLIB library. 3. We applied the same method on LAMOST commissioning observations and we prepared stellar spectral templates for the future surveys. 4. We searched new metal-poor (hearafter MP) stellar candidates from LAMOST commissioning observations. The result of these MP star candidates is a prelude to the feasibility of LAMOST's capability on searching and enlarging the sample of MP stars in the Milky Way

This dissertation describes a new detailed abundance study of field red horizontal branch stars, RR Lyrae stars and blue horizontal branch stars. To carry out this study, we obtained high-resolution and high signal-to-noise ratio echelle spectra at the McDonald observatory and Las Campanas Observatory. In addition, new pulsational emphemerides were derived to analyze the spectra of RR Lyrae stars throughout the pulsational cycles. We find that the abundance ratios are generally consistent with those of field stars of similar metallicity in different evolutionary stages and throughout the pulsational cycles for RR Lyrae stars. We also estimated the red and blue edges of the RR Lyrae instability strip using the derived effective temperatures of RHB and BHB stars. New variations between microturbulence and effective temperature are found among the HB population. For the first time the variation of microturbulence as a function of phase is empirically shown to be similar to the theoretical calculations. Finally, through the study of a rare eclipsing sdB and M dwarf binary, we discovered an unusually low mass for this type of HB star, which observationally proved the existence of a new group of low-mass sdB stars that was theoretically predicted in the past. / text

M dwarfs constitute around 70% of all stars in the local Galaxy. Their multitude together with their long main-sequence lifetimes make them important for studies of global properties of the Galaxy such as the initial mass function or the structure and kinematics of stellar populations. In addition, the exoplanet community is showing an increasing interest for those small, cold stars. However, very few M dwarfs are well characterized, and in the case of exoplanetary systems the stellar parameters have a direct influence on the derived planet properties. Stellar parameters of M dwarfs are difficult to determine because of their low surface temperatures that result in an optical spectrum dominated by molecular lines. Most previous works have therefore relied on empirical calibrations. High-resolution spectrographs operating in the infrared, a wavelength region less affected by molecular lines, have recently opened up a new window for the investigation of M dwarfs. In the two first papers of this thesis we have shown that we can determine the metallicity, and in some cases the effective temperature, using synthetic spectral fitting with improved accuracy. This method is time consuming and therefore not practical or even feasible for studies of large samples of M dwarfs. When comparing our results from the high-resolution studies with available photometric calibrations we find systematic differences. In the third paper we therefore used our sample to determine a new photometric metallicity calibration. Compared to previous calibrations our new photometric calibration shows improved statistical characteristics, and our calibration gives similar results as spectroscopic calibrations. In a comparison with theoretical calculations we find a good agreement of the shapes and slopes of iso-metallicity lines with our empirical relation. Applying the photometric calibration to a sample of M dwarfs with confirmed exoplanets we find a possible giant planet-metallicity correlation for M dwarfs.

Dans le cadre de ma thèse, j'ai effectué l'analyse spectrale d'un échantillon massif de spectres stellaires dans le contexte du programme exoplanète de la mission CoRoT. J'ai tiré avantage des instruments Flames/GIRAFFE pour observer près de 2000 étoiles dans le but de comprendre les populations stellaires dans les champs CoRoT. Dans ce but, j'ai implémenté, calibré, testé, et appliqué une chaîne de traitement et de réduction fiable et efficace afin de réduire et d'analyser automatiquement (en utilisant l'algorithme de paramétrisation MATISSE, Gazzano et al. 2010) un large échantillon de spectres stellaires. J'ai déterminé la vitesse barycentrique radiale, une estimation de la vitesse de rotation projetée sur la ligne de visée, la température effective, la gravité de surface, de la métallicité global et l'enrichissement des éléments par rapport au fer pour 1 227 étoiles dans trois des champs CoRoT. Ainsi, j'ai construit un des premiers échantillons affranchis de biais de sélection pour toute étude concernant la relation planète métallicité dans les champs CoRoT et démontré que le nombre des étoiles naines a été généralement sous-estimé par la classification photométrique (Exo-Dat, Deleuil et al. 2009). J'ai appliqué la relation reliant le nombre de planètes détectées à la métallicité de l'étoile hôte (Udry & Santos 2007), parfaitement en accord avec le nombre actuel de détection planétaire dans les champs CoRoT correspondant (Gazzano et al. 2010). En utilisant les paramètres atmosphériques MATISSE, nous avons déterminé les distances et nous les avons combinées avec des informations cinématiques (les mouvements propres du catalogue PPMXL - Roeser et al. (2010), et l'astrométrie). Nous avons dérivé les composantes de cinématique Galactique : position et vitesse. Cela m'a permis d'étudier les populations stellaires dans les champs CoRoT /exoplanète et de quantifier le gradient de métallicité dans la Galaxie (Gazzano et al. En préparation). J'ai par ailleurs montré que les étoiles à planètes détectées dans les champs considérés pour ma thèse sont exclusivement des étoiles de disque mince. J'ai également participé au suivi des candidats planète CoRoT à l'aide de spectroscopie à haute résolution. J'ai effectué l'analyse spectrale, avec le logiciel VWA (Bruntt et al. 2010b,a), des étoiles hôtes pour la mission spatiale CoRoT. Ces études ont conduit à la détermination des paramètres fondamentaux de l'étoile, qui est une étape indispensable pour la caractérisation complète de la planète. / During my Ph.D., I performed the spectral analysis of a massive sample of stellar spectra in the context of the CoRoT /Exoplanet mission. We took advantage of the Flames/GIRAFFE multi-fibre instrument to observe almost 2 000 stars with the aim of understanding the stellar populations in the CoRoT fields. To these purposes, I implemented, calibrated, tested and applied an automatic pipeline to reduce and analyse automatically (using the parameterization algorithm MATISSE, Gazzano et al. 2010) a large sample of stellar spectra. I derived the barycentric radial velocity, an estimate of the rotational velocity projected on the line of sight, the effective temperature, the surface gravity, the overall metallicity and the -enhancement for 1227 stars in three of the CoRoT fields. Hence, I built one of the first unbiased samples for any study regarding planet metallicity relationship in the CoRoT fields and demonstrated that the amount of dwarf stars was generally underestimated by the photometric classification (in Exo-Dat, Deleuil et al. 2009). I applied the relationship linking the number of planets as a function of the metallicity of the host star (Udry & Santos 2007), totally in agreement with the current number of planetary detection in the corresponding CoRoT fields (Gazzano et al. 2010). Using MATISSE atmospheric parameters, we determined distances and combining them with kinematics information (proper motions from PPMXL catalogue - Roeser et al. (2010) and astrometry), we derived Galactic kinematics components : position, velocities and orbits. This allowed me to study the stellar populations in the CoRoT /Exoplanet fields and quantify the metallicity gradient in the Galaxy (Gazzano et al. in preparation). I also participated to the spectroscopic follow-up observations of CoRoT planetary candidates with high resolution spectroscopy, to the spectroscopic analysis, with the VWA software (Bruntt et al. 2010b,a), of planet hosting stars for the CoRoT space mission and to their characterisation and publication. Indeed, the determination of the fundamental parameters of the star is a mandatory step for the complete characterisation of the planet.

Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013, ApJ, 762, L8).

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