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.

Debris disks are a fundamental component of exoplanetary systems. Understanding their relationship with host stars and neighboring planets can help contextualize the evolution of exoplanetary systems. In order to further that goal, this thesis addresses some extreme outlier examples of debris disk systems. First, the highly asymmetric debris disk around HD 111520 is resolved and analyzed at multiple wavelengths to create a self-consistent model of the disk thermal emission and scattered light. The best-fit model is proposed to be an asymmetric disk from a recent collision of large, icy bodies on one side of the disk. In contrast, most debris disks are thought to be in a steady collisional cascade and this disk model could represent a relatively rare event in the creation of debris disks. Secondly, an optical spectroscopic survey of stars is conducted on stars where far-infrared observations exist to detect the presence of debris disks. Specifically, AF-type stars are targeted in order to provide context regarding the Lambda Boo phenomenon, where stars are found to be specifically refractory metal-poor. One mechanism for this was hypothesized to be from planetary scattering of debris disks, causing the accretion of volatiles from comets. The findings were that over the entire unbiased sample, stars which were refractory metal poor tended to be the stars with brightest debris disks. This supports a planet-disk hypothesis underlying the accretion of volatile gases, since debris disks undergoing active planetary stirring are brighter. This would mean about 13\% of stars with debris disk are undergoing strong planetary scattering based on the occurrence rate of Lambda Boo stars relative to debris disk stars. These two tacks in our observational understanding of these extreme examples of debris disks provide constraints on the volatility at work. / Graduate

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.

abstract: The only elements that were made in significant quantity during the Big Bang were hydrogen and helium, and to a lesser extent lithium. Depending on the initial mass of a star, it may eject some or all of the unique, newly formed elements into the interstellar medium. The enriched gas later collapses into new stars, which are able to form heavier elements due to the presence of the new elements. When we observe the abundances in a stellar regions, we are able to glean the astrophysical phenomena that occurred prior to its formation. I compile spectroscopic abundance data from 49 literature sources for 46 elements across 2836 stars in the solar neighborhood, within 150 pc of the Sun, to produce the Hypatia Catalog. I analyze the variability of the spread in abundance measurements reported for the same star by different surveys, the corresponding stellar atmosphere parameters adopted by various abundance determination methods, and the effect of normalizing all abundances to the same solar scale. The resulting abundance ratios [X/Fe] as a function of [Fe/H] are consistent with stellar nucleosynthetic processes and known Galactic thin-disk trends. I analyze the element abundances for 204 known exoplanet host-stars. In general, I find that exoplanet host-stars are not enriched more than the surrounding population of stars, with the exception of iron. I examine the stellar abundances with respect to both stellar and planetary physical properties, such as orbital period, eccentricity, planetary mass, stellar mass, and stellar color. My data confirms that exoplanet hosts are enriched in [Fe/H] but not in the refractory elements, per the self-enrichment theory for stellar composition. Lastly, I apply the Hypatia Catalog to the Catalog of Potentially Habitable Stellar Systems in order to investigate the abundances in the 1224 overlapping stars. By looking at stars similar to the Sun with respect to six bio-essential elements, I created maps that have located two ``habitability windows'' on the sky: (20.6hr, -4.8deg) and (22.6hr, -48.5deg). These windows may be of use in future targeted or beamed searches. / Dissertation/Thesis / Ph.D. Astrophysics 2012

The discovery of exoplanets represents one of the greatest scientific revolutions in history, and exoplanetary science has rapidly become uniquely positioned to address profound questions about the origins of life, and about humanity's place (and future) in the cosmos. Since the discovery of the first exoplanet over two decades ago, the radial velocity (RV) method has been one of the most productive techniques for discovering new planets. It has also become indispensable for characterising exoplanets detected via other techniques, notably transit photometry. Unfortunately, signals intrinsic to stars themselves - especially magnetic activity signals - can induce RV variations that can drown out or even mimic planetary signals. Modelling and thus mitigating these signals is notoriously difficult, which represents a major obstacle to using next-generation instruments to detect lower mass planets, planets with longer periods, and planets around more magnetically-active stars. Enter Gaussian processes (GPs), which have a number of features that make them very well suited to the joint modelling of stochastic activity processes and dynamical (e.g. planetary) signals. In this thesis, I leverage GPs to enable the study of smaller planets around a wider variety of stars than has previously been possible. In particular, I develop a principled and sophisticated Bayesian framework, based on GPs, for modelling RV time series jointly with ancillary activity-sensitive proxies, thus allowing activity signals to be constrained and disentangled from genuine planetary signals. I show that my framework succeeds even in cases where existing techniques would fail to detect planets, e.g. the case of a weak planetary signal with period identical to its host star's rotation period. In a first application of the framework, I demonstrate that Alpha Centauri Bb - until 2016, thought to be the closest exoplanet to Earth, and also the lowest minimum-mass exoplanet around a Sun-like star - was, in fact, an astrophysical false positive. Next, I use the framework to re-characterise the well-studied Kepler-10 system, thereby resolving a mystery surrounding the mass of planet Kepler-10c. I also use the framework to help discover or characterise various exoplanets. Finally, the activity modelling framework aside, I also present in outline form a few promising applications of GPs in the context of modelling stellar signals and studying exoplanets, viz. GPs for (i) enhanced characterisation of stellar rotation; (ii) generating realistic synthetic observations, and modelling in a systematic way the effects of an observing window function; and (iii) ultra-precise extraction of RV shifts directly from observed spectra, without requiring template cross-correlation.

Les hétérogénéités de surface des étoiles actives, telles que les taches et les facules, peuvent compliquer l'interprétation des spectres de transmission en introduisant des caractéristiques spectrales qui chevauchent celles d'atmosphère d'exoplanètes. Les courbes de lumière de transit d'HAT-P-18\(\,\)b et de WASP-52\(\,\)b, observées avec le mode SOSS de l'instrument NIRISS à bord du JWST, sont déformées par des occultations de taches. Avant le déploiement du JWST, ces régions actives étaient souvent simplement masquées, toutefois ceci peut mener à des mesures incorrectes des paramètres du transit. J'ai adapté et implémenté \(\texttt{spotrod}\), un modèle de transit avec occultation de taches, dans l'outil \(\texttt{Juliet}\) pour inférer conjointement les paramètres du transit et des taches occultées. J'ai ainsi ajusté les courbes de lumière de transit de ces deux Jupiters chaudes et récupéré la position de chaque tache, leur rayon et leur spectre de contraste, c'est-à-dire le rapport du flux de la tache sur le flux stellaire. J'ai contraint la température des taches et leur gravité de surface (pour prendre en compte les effets du champ magnétique local) en ajustant chaque spectre de contraste avec des spectres de modèles stellaires PHOENIX. Cependant, un certain degré de dégénérescence est présent, conduisant à une solution plus probable pour chaque tache, mais aussi à d'autres solutions qui ne peuvent être exclues. Le spectre de transmission d'HAT-P-18\(\,\)b nous a permis de détecter de l'H\(_2\)O (12,5\(\,\sigma\)) avec une abondance sub-solaire de \(\log\) H\(_2\)O \(\approx\) -4,4 \(\pm\) 0,3, des nuages (7,4\(\,\sigma\)) et du CO\(_2\) (7,3\(\,\sigma\)) dans l'atmosphère planétaire ainsi que des régions actives non occultées (5,8\(\,\sigma\)) qui imitent une pente de diffusion Rayleigh. / Surface heterogeneities on active stars, such as starspots and faculae, can complicate the interpretation of transmission spectra and introduce spectral features that overlap those of exoplanetary atmospheres. The transit light curves of HAT-P-18\(\,\)b and WASP-52\(\,\)b, observed in the SOSS mode of the NIRISS instrument aboard the JWST, are deformed by spot-crossings. These active regions were often simply masked before the launch of the JWST; however, this can prevent the correct measure of transit parameters. I adapted and implemented \(\texttt{spotrod}\), a model for transits of spotted stars, into the \(\texttt{Juliet}\) tool to simultaneously infer the transit and occulted starspots parameters. I fitted the transit light curves of these two hot Jupiters and retrieved for each spot its position, radius and spot-to-stellar flux contrast spectrum. I constrained the spots' temperature and surface gravity \(-\) attempting to capture the effects of the local magnetic pressure \(-\) by fitting each contrast spectrum with PHOENIX stellar model spectra. However, some degree of degeneracy is present, leading to a most likely solution for each starspot and other solutions that cannot be excluded. The transmission spectrum of HAT-P-18\(\,\)b enabled us to detect H\(_2\)O (12.5\(\,\sigma\)) with a sub-solar abundance of \(\log\) H\(_2\)O \(\approx\) -4.4 \(\pm\) 0.3, a cloud deck (7.4\(\,\sigma\)) and CO\(_2\) (7.3\(\,\sigma\)) in the planetary atmosphere as well as unocculted active regions (5.8\(\,\sigma\)) which mimic a Rayleigh scattering slope.