Os modelos atuais sobre formação planetária indicam que os planetas gasosos gigantes formam-se em escalas de tempo de ~ 10 Manos, inferiores à dos pequenos, rochosos, de ~ 30 Manos (Zuckerman & Song 2004). Um teste simples desses modelos de formação seria procurar planetas em torno de estrelas jovens de várias idades: nos sistemas mais jovens não dever-se-ia detectar objetos telúricos, que só apareceriam em torno de estrelas relativamente mais velhas. Os satélites CoRoT e Kepler, que se encontram em pleno regime de observações, vêm descobrindo exoplanetas pelo método dos trânsitos, sendo capaz de detectar corpos de dimensões terrestres. O objetivo deste trabalho, é selecionar sistemas planetários jovens de várias idades para serem observados pelo dois satélites, a fim de testar as escalas de tempo de formação de planetas rochosos e gasosos. Para atingir esse objetivo foi necessário entender como a idade pode ser estimada para um grupo de estrelas (aglomerado aberto ou associação), utilizando-se, por exemplo, dados sobre abundâncias químicas do lítio dos objetos. Isso é possível devido à facilidade do lítio em ser destruído na fase pré-sequencia principal, a temperaturas superiores 2,5 10^6 K. Um levantamento amostral da abundância do lítio em função da temperatura, para estrelas pertecentes a um grupo, gera um padrão de depleção do lítio, que permite gerar um modelo (da Silva et al. 2009) qualitativo para se obter idades de associações estelares. Para que o propósito deste trabalho fosse alcançado, foram utilizados os bancos de dados de objetos jovens existentes em associações com idades bem determinadas de (Torres et al. 2008), e o catálogo DAML de (Dias et al. 2002) de aglomerados abertos. A seleção dos dados para cada satélite produziu resultados bem diferentes. Para o CoRoT, a análise do banco de dados de associações retornou resultados com uma associação no centro galáctico e outra de ~ 70 Manos no anti-centro; por outro lado, no campo do Kepler, não se encontrou objetos jovens que possibilitassem atender os objetivos deste trabalho. Na análise do catálogo DAML de aglomerados abertos, surgiram muitos candidatos-alvo para observações. Para o CoRoT, foi possível concluir que os aglomerados NGC 2244 de 7,87 Manos, NGC 2264 de 8,99 Manos, Collinder 107 de 10 Manos, Collinder 96 de 10,74 Manos, e NGC 2302 de 12,02 Manos contêm alvos onde deve-se encontrar somente planetas gigantes gasosos em estágio inicial e/ou final de formação, de acordo com o capítulo 1. Os aglomerados relativamente mais velhos, onde devem-se encontrar planetas rochosos e gasosos são: NGC 6755 de 52,36 Manos, Basel 1 de 78,16 Manos, NGC 6694 de 85,31 Manos, NGC 2186 de 54,70 Manos, NGC 2422 de 72,61 Manos e Bochum 3 de 77,62 Manos. Portanto a etapa de seleção de alvos a serem observados pelo CoRoT foi feita, e com isso, os eventuais resultados observacionais servirão de teste para as escalas de tempo de formação planetária propostos nos modelos correntes. Para o Kepler, não foi encontrado nenhum membro de aglomerado jovem observável em seu campo de visibilidade e seu intervalo de magnitudes. / Current models of planetary formation suggest that the giant gaseous planets are formed in time scales of ~ 10 Myr, less than the rocky ones, in time scales of ~ 30 Myr (Zuckerman & Song 2004). A simple test of these models of formation it would look for planets around young stars of various ages: in younger systems it should not detect terrestrial objects, which only appear around stars relatively older. CoRoT and Kepler satellites, which are at full system of observations, have been discovering exoplanets by the method of transits, being able to detect Earth-size bodies. The goal of this work is to select young planetary systems of various ages to be observed by the two satellites, in order to test the time scales of formation of rocky and gaseous planets. To achieve this goal it was necessary to understand how age can be estimated for a group of stars (open cluster or association), using, for example, data on chemical abundances of lithium objects. This is possible because of the ease of lithium to be destroyed in the pre-main sequence, at temperatures above 2,5 10^6 K. A sample survey of the abundance of lithium as a function of temperature for stars belonging to a group, generates a lithium depletion pattern, which creates a qualitatively model (da Silva et al. 2009) to obtain ages of star associations. For the purpose of this study was reached, the databases of objects in youth associations with well-determined ages from (Torres et al. 2008) was used, as well, the catalog DAML from (Dias et al. 2002) of open clusters. The selection of data for each satellite has produced quite different results. For CoRoT, the analysis of the database of associations returned results with a association in the galactic center and another with ~ 70 Myr in the anti-Galactic center, on the other hand, in the field of Kepler did not find young objects that would enable meet the goals of this work.. In the analysis of DAML catalog of open clusters have emerged many candidates targeted for observations. For the CoRoT was concluded that the clusters NGC 2244 of 7,87 Myr, NGC 2264 of 8,99 Myr, Collinder 107 of 10 Myr, Collinder 96 of 10,74 Myr, and NGC 2302 of 12,02 Myr contain targets where should be found only gas giant planets in the early stage and/or end of formation, in accordance with Chapter 1. The relatively older clusters, where they must be found rocky and gaseous planets are: NGC 6755 of 52,36 Myr, Basel 1 of 78,16 Myr, NGC 6694 of 85,31 Myr, NGC 2186 of 54,70 Myr, NGC 2422 of 72,61 Myr e Bochum 3 de 77,62 Myr. Therefore the step of selecting targets to be observed by the CoRoT was made, and thus, any observational results serve as a test for the timescales of planet formation proposed in the current models. For Kepler, it did not find any member of young cluster observed in its field of vision and its range of magnitudes.

Since the first successful measurements of stellar trigonometric parallax in the 1830s, the study of nearby stars has focused on the highest proper motion stars (mu > 0.18"/yr). Those high proper motion stars have formed the backbone of the last 150 years of study of the Solar Neighborhood and the composition of the Galaxy. Statistically speaking, though, there is a population of stars that will have low proper motions when their space motions have been projected onto the sky. At the same time, over the last twenty years, populations of relatively young stars (less than ~100 Myr), most of them with low proper motions, have been revealed near (<100 >pc) the Sun. This dissertation is the result of two related projects: A photometric search for nearby (<25 >pc) southern-hemisphere M dwarf stars with low proper motions (mu < 0.18"/yr), and a search for nearby (

In Chapter 2 of this thesis I present my database of molecular absorption cross sections. These were developed using public molecular transition line-lists (from the ExoMol group). I use them to find limitations in the modelling of exoplanet atmospheres due to pressure broadening. Pressure broadening, where collisions between molecules in atmospheres cause a Lorentzian broadening of molecular transitional lines, is little understood in the field. In this chapter I consider its effects on real exoplanet atmosphere observations, both with current and future instruments. I show that pressure broadening may affect future observations of exoplanets in the JWST era. Pressure broadening primarily affects cooler, small exoplanets such as Earth analogues. In Chapter 3 I present the pipeline I have developed to reduce HST WFC3 spectra of exoplanet hosts during transits to create transmission spectra. This code corrects several instrumental systematics, from varying dark signal in the detector to subpixel shifts in the target position over time. By creating a pipeline to process all targets, regardless of observing strategy, systematics are dealt with uniformly and different planets’ spectra can be meaningfully compared. I show that the height of the water feature in 30 unique exoplanets’ transmission spectra is strongly correlated with the most simplistic absorption model. I use this to predict a list of the best future targets for observations with HST WFC3 to find water. In Chapter 4 I discuss my work with the stellar spectra from WFC3, which utilise the sub-pixel shifts in target position to oversample the spectra and increase the resolution. I have compared these exoplanet host stellar spectra with stellar models to investigate how well stellar atmosphere models describe the near IR. I find a small discrepancy in temperature when WFC3 alone is used to assess the stellar temperature, particularly with cooler stars. I attribute this firstly to an error in the WFC3 sensitivity curve and secondly to an inaccuracy in models of cool, small stars due to molecular absorption. In Chapter 5 I present my work on K2 light curve data using machine learning to find young stellar objects that display unusual, transit-like behaviour. These objects are known as dipper stars due to their distinctive occultations with depths of 10-50% in flux and very fast orbital periods of a few hours to a few days. Such large occultations are difficult to explain and are currently attributed to material at the inner edge of the protoplanetary disk. This behaviour is often variable and aperiodic, suggesting that the occulting material is changing in morphology on the time scale of a single orbit. Using python’s scikit-learn I have developed a code that utilises a Random Forest algorithm to classify stars in K2 Campaign Field 2 and distinguish these objects from other types of variables, such as eclipsing binaries and pulsating stars. This method has proved very successful and has allowed me to nearly quadruple the number of known dipper candidates in the Upper Scorpius and Rho Ophiuchus regions.

Os modelos atuais sobre formação planetária indicam que os planetas gasosos gigantes formam-se em escalas de tempo de ~ 10 Manos, inferiores à dos pequenos, rochosos, de ~ 30 Manos (Zuckerman & Song 2004). Um teste simples desses modelos de formação seria procurar planetas em torno de estrelas jovens de várias idades: nos sistemas mais jovens não dever-se-ia detectar objetos telúricos, que só apareceriam em torno de estrelas relativamente mais velhas. Os satélites CoRoT e Kepler, que se encontram em pleno regime de observações, vêm descobrindo exoplanetas pelo método dos trânsitos, sendo capaz de detectar corpos de dimensões terrestres. O objetivo deste trabalho, é selecionar sistemas planetários jovens de várias idades para serem observados pelo dois satélites, a fim de testar as escalas de tempo de formação de planetas rochosos e gasosos. Para atingir esse objetivo foi necessário entender como a idade pode ser estimada para um grupo de estrelas (aglomerado aberto ou associação), utilizando-se, por exemplo, dados sobre abundâncias químicas do lítio dos objetos. Isso é possível devido à facilidade do lítio em ser destruído na fase pré-sequencia principal, a temperaturas superiores 2,5 10^6 K. Um levantamento amostral da abundância do lítio em função da temperatura, para estrelas pertecentes a um grupo, gera um padrão de depleção do lítio, que permite gerar um modelo (da Silva et al. 2009) qualitativo para se obter idades de associações estelares. Para que o propósito deste trabalho fosse alcançado, foram utilizados os bancos de dados de objetos jovens existentes em associações com idades bem determinadas de (Torres et al. 2008), e o catálogo DAML de (Dias et al. 2002) de aglomerados abertos. A seleção dos dados para cada satélite produziu resultados bem diferentes. Para o CoRoT, a análise do banco de dados de associações retornou resultados com uma associação no centro galáctico e outra de ~ 70 Manos no anti-centro; por outro lado, no campo do Kepler, não se encontrou objetos jovens que possibilitassem atender os objetivos deste trabalho. Na análise do catálogo DAML de aglomerados abertos, surgiram muitos candidatos-alvo para observações. Para o CoRoT, foi possível concluir que os aglomerados NGC 2244 de 7,87 Manos, NGC 2264 de 8,99 Manos, Collinder 107 de 10 Manos, Collinder 96 de 10,74 Manos, e NGC 2302 de 12,02 Manos contêm alvos onde deve-se encontrar somente planetas gigantes gasosos em estágio inicial e/ou final de formação, de acordo com o capítulo 1. Os aglomerados relativamente mais velhos, onde devem-se encontrar planetas rochosos e gasosos são: NGC 6755 de 52,36 Manos, Basel 1 de 78,16 Manos, NGC 6694 de 85,31 Manos, NGC 2186 de 54,70 Manos, NGC 2422 de 72,61 Manos e Bochum 3 de 77,62 Manos. Portanto a etapa de seleção de alvos a serem observados pelo CoRoT foi feita, e com isso, os eventuais resultados observacionais servirão de teste para as escalas de tempo de formação planetária propostos nos modelos correntes. Para o Kepler, não foi encontrado nenhum membro de aglomerado jovem observável em seu campo de visibilidade e seu intervalo de magnitudes. / Current models of planetary formation suggest that the giant gaseous planets are formed in time scales of ~ 10 Myr, less than the rocky ones, in time scales of ~ 30 Myr (Zuckerman & Song 2004). A simple test of these models of formation it would look for planets around young stars of various ages: in younger systems it should not detect terrestrial objects, which only appear around stars relatively older. CoRoT and Kepler satellites, which are at full system of observations, have been discovering exoplanets by the method of transits, being able to detect Earth-size bodies. The goal of this work is to select young planetary systems of various ages to be observed by the two satellites, in order to test the time scales of formation of rocky and gaseous planets. To achieve this goal it was necessary to understand how age can be estimated for a group of stars (open cluster or association), using, for example, data on chemical abundances of lithium objects. This is possible because of the ease of lithium to be destroyed in the pre-main sequence, at temperatures above 2,5 10^6 K. A sample survey of the abundance of lithium as a function of temperature for stars belonging to a group, generates a lithium depletion pattern, which creates a qualitatively model (da Silva et al. 2009) to obtain ages of star associations. For the purpose of this study was reached, the databases of objects in youth associations with well-determined ages from (Torres et al. 2008) was used, as well, the catalog DAML from (Dias et al. 2002) of open clusters. The selection of data for each satellite has produced quite different results. For CoRoT, the analysis of the database of associations returned results with a association in the galactic center and another with ~ 70 Myr in the anti-Galactic center, on the other hand, in the field of Kepler did not find young objects that would enable meet the goals of this work.. In the analysis of DAML catalog of open clusters have emerged many candidates targeted for observations. For the CoRoT was concluded that the clusters NGC 2244 of 7,87 Myr, NGC 2264 of 8,99 Myr, Collinder 107 of 10 Myr, Collinder 96 of 10,74 Myr, and NGC 2302 of 12,02 Myr contain targets where should be found only gas giant planets in the early stage and/or end of formation, in accordance with Chapter 1. The relatively older clusters, where they must be found rocky and gaseous planets are: NGC 6755 of 52,36 Myr, Basel 1 of 78,16 Myr, NGC 6694 of 85,31 Myr, NGC 2186 of 54,70 Myr, NGC 2422 of 72,61 Myr e Bochum 3 de 77,62 Myr. Therefore the step of selecting targets to be observed by the CoRoT was made, and thus, any observational results serve as a test for the timescales of planet formation proposed in the current models. For Kepler, it did not find any member of young cluster observed in its field of vision and its range of magnitudes.

No description available.

This thesis presents both a study of the cluster-scale environments in which massive stars form, investigating in particular how the ionized gas in these regions relates to the molecular star-forming material, as well as detailed studies of two luminous forming stars, AFGL 2591 and IRAS 20126+4104, to determine whether they are forming similarly to their low-mass counterparts. The results of this work include the identification of 35 HII regions (20 newly discovered) via a radio continuum survey of ionized gas towards 31 molecular cluster-forming clumps. The observed ionized gas was found to be preferentially associated with the clumps, which were shown to have a range of evolutionary stages. The massive star formation efficiency was determined for the clumps with associated ionized gas, and a relationship was found between the mass of the clumps and the mass of their embedded massive stars. By modelling the SEDs and images of AFGL 2591 and IRAS 20126+4104, it was found that the geometry of their circumstellar material was generally consistent with an envelope plus disk, similar to that expected for low-mass protostars. However, within the central ~1800 AU, the mid-IR images of IRAS 20126+4104 were better described by only a flattened envelope, suggesting that the radiation from IRAS 20126+4104 may be affecting the regions closest to the star. Observations of the ionized and molecular gas towards AFGL 2591 were carried out, and a photoionization code was developed to interpret these observations. The results showed that the observed 3.6 cm emission is likely to be produced by both a shock-ionized jet and a hypercompact HII region that does not appear to have disrupted the jet or the large-scale circumstellar environment. In addition, the C¹⁸O(1-0) emission observed towards AFGL2591 traces the densest parts of the outflow, with the blue-shifted emission exhibiting many of the properties of the outflows from low-mass protostars.

In this dissertation, we conduct a census and assessment of the nearest young Sun-like stars and investigate the potential for finding giant planets orbiting spotted stars using the radial velocity (RV) method at optical and near-infrared wavelengths. Based in part on new spectroscopic measurements conducted here, we have assembled a complete list of 129 young (<150 >Myr), nearby Sun-like stars and their fundamental parameters, including rotational and multiplicity information. We also provide a statistical analysis of their stellar parameters, including projected rotational velocity and inclination. Sixteen of these stars have no close companions and have low projected rotational velocities (vsini/s) that are ideal for precision RV planet searches. Seven of these rotate nearly edge-on and are ideal targets for upcoming transiting planet searches, assuming low obliquity. We conduct precision RV planet search of 7 young Sun-like stars using the TRES spectrograph, mounted on the 1.5-m Tillinghast Reflector at the Fred L. Whipple Observatory, and with the SOPHIE spectrograph, mounted on the 1.93-m Telescope at the Observatoire de Haute Provence; we achieve a precision of 10 m/s for both. Four stars are identified as having larger RV variations that are periodic, possibly caused by an orbiting companion. However, the RV variations are correlated with asymmetries in the spectral absorption features, which instead suggests that the variations are caused by spots. Nevertheless our observations provide new independent measures of the rotation periods of these stars. Through this analysis we tentatively confirm the planetary companion around BD+20 1790 in the presence of activity. We additionally investigate the use of comparing red orders of the optical spectrum to the blue orders in order to distinguish spots from planets; we find that this method can be effective for observations that span the full wavelength range of the optical. We also investigate our detection limits at optical wavelengths and find that we are sensitive to over 90% of short period giant planets. Next, we assemble the stellar jitter measurements of our stars with previous studies of all Sun-like stars younger than 1 Gyr to investigate how stellar jitter declines with stellar age. We find that stellar jitter decreases with stellar age as t^(0.53±0.13), similar to the relationship between stellar rotation period and stellar age. The implication is that it will be diffcult to find planets orbiting stars younger than 100 Myr without using techniques that mitigate star spot noise. Furthermore, we present a near-infrared RV search for giant planets orbiting 8 stars observed with CSHELL at the NASA Infrared Telescope Facility (IRTF). Because of the limited wavelength coverage (29 ̊A) and older (1980s) detector technology, the achieved precision of 200 m/s inhibits finding the majority of exoplanets, but is nevertheless sufficient to identify short-period brown dwarfs for these stars. We also analyze our detection limits at IR wavelengths and find that we are only sensitive to roughly 50% of short period giant planets. Finally, we present a new orbital solution for V835 Her, a spectroscopic binary with a 3 day orbital period.

A broad near-infrared photometric survey is conducted of 1678 stars in the direction of the $\rho$ Ophiuchi ($\rho$ Oph) star forming region using data from the 2MASS Calibration Database. The survey involves up to 1584 photometric measurements in the \emph{J}, \emph{H} and \emph{K$_{s}$} bands with an $\sim$1 day cadence spanning 2.5 years. Identified are 101 variable stars with $\Delta$\emph{K$_{s}$} band amplitudes from 0.044 to 2.31 mag and $\Delta$(\emph{J}-\emph{K$_{s}$}) color amplitudes ranging from 0.053 to 1.47 mag. Of the 72 $\rho$ Oph star cluster members, 79$\%$ are variable; in addition, 22 variable stars are identified as candidate members. The variability is categorized as periodic, long timescale, or irregular based on the \emph{K$_{s}$} time series morphology. The dominant variability mechanisms are assigned based on the correlation between the stellar color and single band variability. Periodic signals are found in 32 variable stars with periods between 0.49 to 92 days. The most common variability mechanism among these stars is rotational modulation of cool starspots. Periodic eclipse-like variability is identified in 6 stars with periods ranging from 3 to 8 days; in these cases the variability mechanism may be warped circumstellar material driven by a hot proto-Jupiter. Aperiodic, long time scale variability is identified in 31 stars with time series ranging from 64 to 790 days. The variability mechanism is split evenly between either variable extinction or mass accretion. The remaining 40 stars exhibit sporadic, aperiodic variability with no discernible time scale or variability mechanism. Interferometric images of the active giant $\lambda$ Andromedae ($\lambda$ And) were obtained for 27 epochs spanning November. 2007 to September, 2011. The \emph{H} band angular diameter and limb darkening coefficient of $\lambda$ And are 2.777 $\pm$ 0.027 mas and 0.241 $\pm$ 0.014, respectively. Starspot properties are extracted via a parametric model and an image reconstruction program. High fidelity images are obtained from the 2009, 2010, and 2011 data sets. Stellar rotation, consistent with the photometrically determined period, is traced via starspot motion in 2010 and 2011. The orientation of $\lambda$ And is fully characterized with a sky position angle and inclination angle of 23$\degree$ and 78$\degree$, respectively.

Le moment cinétique d’une étoile, comme sa masse ou sa composition chimique, est l’une de ses propriétés fondamentales, l’un de celles qui varient à cours du temps et influent sur la structure de l’étoile. Celui-ci peut être global, on l’observe alors à travers la vitesse de rotation de surface d’une étoile, ou local, auquel cas il nous faut sonder l’intérieur stellaire et étudier les processus de redistribution au sein des régions internes du moment cinétique. Au cours de cette thèse dans le cadre du projet ToUpiES, nous nous sommes intéressés en particulier à l’évolution du moment cinétique des étoiles de faible masse au cours de leur jeunesse, qui est une période critique de leur vie en ce qui concerne l’impact et l’évolution du moment cinétique. Nous avons d’abord inclus au sein du code d’évolution STAREVOL les prescriptions les plus à jour pour l’extraction du moment cinétique par les vents magnétisés. L’étude systématique des combinaisons de ce freinage avec différentes prescriptions existantes pour le traitement de la turbulence horizontale et verticale dans la zone radiative des étoiles, nous a permis de sélectionner un jeu de prescriptions capable de reproduire, les périodes de rotation dans les amas ouverts pour une étoile de type solaire. Nous comparons ensuite l’application de ces processus de transport et d’extraction du moment cinétique à un modèle de 1, 2 masse solaire, aux autres processus jugés potentiellement efficaces pour transport le moment cinétique à ce jour (ondes internes de gravités, instabilité MHD de Tayler-Spruit, modes de gravités). Cela nous a permis de présenter dans chacun des cas les spécificités du profil de rotation prédit par ces différents modes de transport. Puis, nous avons mis en place un modèle rotationnel fonctionnel adapté à l’ensemble des étoiles de faible masse, permettant entre autre de reproduire les périodes de rotation observées dans les amas jeunes pour les étoiles de faible masse (avec une masse comprise entre 0, 2 et 1, 1 M⊙). Ceci a donné lieu à une grille de modèle d’évolution unique à ce jour. Enfin, cette grille a été utilisée dans le cadre de travaux dans différents domaines, tels que l’impact de l’évolution stellaire sur l’habitabilité d’un système, la caractérisation d’étoiles-hôte ou encore l’étude de l’évolution de la topologie magnétique au cours des phases jeunes. / The angular momentum content of a star, as its mass or its chemical composition is one of the fundamental properties of a star, one of those that evolves with time and modify the stellar structure. The angular momentum can be studied as a global property, we can then observe it through the surface rotation velocity, or a local property that vary inside the star, we therefore have to probe the stellar radiation zone and study the secular angular momentum redistribution processes that happen in this region. During this PhD, in the frame of the ToUpiES project, we have been especially interested in the evolution of the young low-mass stars angular momentum, since this phase of evolution is critical regarding the evolution of extraction and redistribution angular momentum processes. First, we included in the STAREVOL evolution code the most up-to-date prescription for the wind-driven angular momentum extraction. We led a systematic study of the various combination of this braking with the different existing prescriptions for the treatment of horizontal and vertical turbulent motions in stellar radiative zones. This allows us to select a set of prescription able to reproduce the observed rotation periods in young open clusters for a broad mass-range. Next, we analysed how these prescriptions for extraction and transport of angular momentum behave when applied to a 1.2M⊙ model. We compared the result to what is obtained with other processes estimated as potentially very efficient to redistribute angular momentum (internal gravity waves, MHD Tayler-Spruit instability, gravity modes). This allows us to derive in each case, the specificity of the rotation profiles predicted by the different transport processes. Then, we set up a functional rotational model adapted to almost the entire range low-mass stars, allowing to reproduce the observed low-mass stars rotation periods in young open clusters (with 0, 2M⊙ ≤M≤ 1, 1M⊙). This models can also predict the rotational evolution at different metallicities. Eventually, these models have been used in the frame of various works in different domains such as the characterisation of planet host-stars, the evolution of the magnetic topology during the young stellar phases or even the impact of stellar evolution on the habitability of a planetary system.

Des colonnes d'accrétion relient les étoiles jeunes au disque de gaz et de poussière qui les entoure. De nombreuses études numériques ont montré l'existence d'une structure oscillante de plasma choqué au sein de ces colonnes. Cependant, aucune observation n'est en mesure de confirmer l'existence d'un tel phénomène. Ces simulations s'appuient toutes sur le postulat selon lequel le gaz accrété, suit une fonction de refroidissement optiquement mince. L'objectif principal de mon travail a été de m'affranchir de ce postulat. Après avoir amélioré la description de processus microscopiques importants dans le code 1D RHD ALE AstroLabE, j'ai travaillé sur les tables d'opacités, grandeurs clés dans l'interaction entre le champ de rayonnement et la matière. Les résultats obtenus montrent que l'absorption d'une faible fraction du rayonnement est capable d'affecter significativement la dynamique de la structure de gaz choqué, jusqu'à supprimer le comportement oscillatoire prédit. Je me suis également attaché à modéliser de manière cohérente la structure sur laquelle s'effectue l'accrétion: la chromosphère. J'ai pour cela utilisé un modèle simple d'atmosphère chauffée par des ondes acoustiques dégénérant en chocs. Si la dynamique de l'écoulement reste périodique, moyennant quelques perturbations, la luminosité X présente des modulations d'amplitude relativement modestes. Ce travail illustre l'importance du transfert radiatif dans le processus d'accrétion et d'une description réaliste de ce transfert radiatif. Les méthodes qualitatives que j'ai développées, adaptées à une modélisation 1D, ouvrent la voie à d'autres développements, notamment dans le cadre de simulations à plusieurs dimensions. / Accretion columns connect young stars to the surrounding disk of gas and dust. Numerous numerical studies have predicted quasi-periodic oscillations of the shocked structure at the base of these columns. There is, however, no observational evidence of such feature. These simulations rely on the assumption that accreted gas can be described with an optically thin line cooling function. The main goal of my work has been to go beyond this assumption. I started with the improvement of the description of important microscopic processes included in the 1D ALE RHD code AstroLabE. I worked then on the building of adapted opacity tables, to take into account the coupling between radiation and matter. The results show that even by taking into account the absorption of a small fraction of radiation, the dynamics of the shocked gas structure is significantly affected, and the predicted oscillatory behavior may be suppressed. I have concentrated on the coherent modeling of the stellar chromosphere above which the accretion takes place. For this purpose, I used a model based on acoustic waves heating. Although the chromospheric shock waves perturb the dynamics of accretion (which remains periodic), the computed luminosity presents modulations of relative small amplitude. The work highlights the importance of the radiative transfer in the accretion process on young stars and the necessity of an adequate, physically based, description of the radiative transfer. The methods I have developed in this work will foster developments of multi-dimensional simulations.