Earths and mini Neptunes likely have a wide range of atmospheric compositions, ranging from low molecular mass atmospheres of H-2 to higher molecular atmospheres of water, CO2, N-2, or other species. Here we systematically investigate the effects of atmospheric bulk compositions on temperature and wind distributions for tidally locked sub-Jupiter-sized planets, using an idealized 3D general circulation model (GCM). The bulk composition effects are characterized in the framework of two independent variables: molecular weight and molar heat capacity. The effect of molecular weight dominates. As the molecular weight increases, the atmosphere tends to have a larger day-night temperature contrast, a smaller eastward phase shift in the thermal phase curve, and a smaller zonal wind speed. The width of the equatorial super-rotating jet also becomes narrower, and the "jet core" region, where the zonal-mean jet speed maximizes, moves to a greater pressure level. The zonal-mean zonal wind is more prone to exhibit a latitudinally alternating pattern in a higher molecular weight atmosphere. We also present analytical theories that quantitatively explain the above trends and shed light on the underlying dynamical mechanisms. Those trends might be used to indirectly determine the atmospheric compositions on tidally locked sub-Jupiter-sized planets. The effects of the molar heat capacity are generally small. But if the vertical temperature profile is close to adiabatic, molar heat capacity will play a significant role in controlling the transition from a divergent flow in the upper atmosphere to a jet-dominated flow in the lower atmosphere.

We present 197 planet candidates discovered using data from the first year of the NASA K2 mission (Campaigns 0-4), along with the results of an intensive program of photometric analyses, stellar spectroscopy, high-resolution imaging, and statistical validation. We distill these candidates into sets of 104 validated planets (57 in multi-planet systems), 30 false positives, and 63 remaining candidates. Our validated systems span a range of properties, with median values of R-P = 2.3 R-circle plus, P = 8.6 days, T-eff = 5300 K, and Kp = 12.7 mag. Stellar spectroscopy provides precise stellar and planetary parameters for most of these systems. We show that K2 has increased by 30% the number of small planets known to orbit moderately bright stars (1-4 R-circle plus, Kp = 9-13. mag). Of particular interest are 76 planets smaller than 2 R-circle plus, 15 orbiting stars brighter than Kp = 11.5. mag, 5 receiving Earth-like irradiation levels, and several multi-planet systems-including 4 planets orbiting the M dwarf K2-72 near mean-motion resonances. By quantifying the likelihood that each candidate is a planet we demonstrate that our candidate sample has an overall false positive rate of 15%-30%, with rates substantially lower for small candidates (<2 R-circle plus) and larger for candidates with radii >8 R-circle plus and/or with P < 3 days. Extrapolation of the current planetary yield suggests that K2 will discover between 500 and 1000 planets in its planned four-year mission, assuming sufficient follow-up resources are available. Efficient observing and analysis, together with an organized and coherent follow-up strategy, are essential for maximizing the efficacy of planet-validation efforts for K2, TESS, and future large-scale surveys.

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R'(HK) indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca II lines, broadening b-parameter, and Ca II column density). We find that for relative velocities between the stellar and ISM lines smaller than 30-40 km s(-1) and for ISM Ca II column densities log N-CaII greater than or similar to 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca II column density of log N-CaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R'(HK) value larger than 0.05-0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars.

Context. For over 12 yr, we have carried out a precise radial velocity (RV) survey of a sample of 373 G- and K-giant stars using the Hamilton Echelle Spectrograph at the Lick Observatory. There are, among others, a number of multiple planetary systems in our sample as well as several planetary candidates in stellar binaries. Aims. We aim at detecting and characterizing substellar and stellar companions to the giant star HD 59686 A (HR 2877, HIP 36616). Methods. We obtained high-precision RV measurements of the star HD 59686 A. By fitting a Keplerian model to the periodic changes in the RVs, we can assess the nature of companions in the system. To distinguish between RV variations that are due to non-radial pulsation or stellar spots, we used infrared RVs taken with the CRIRES spectrograph at the Very Large Telescope. Additionally, to characterize the system in more detail, we obtained high-resolution images with LMIRCam at the Large Binocular Telescope. Results. We report the probable discovery of a giant planet with a mass of m(p) sin i = 6.92(-0.24)(+0.18) M-Jup orbiting at a(p) = 1.0860(-0.0007)(+0.0006) aufrom the giant star HD 59686 A. In addition to the planetary signal, we discovered an eccentric (e(B) = 0.729(-0.003)(+0.004)) binary companionwith a mass of m(B) sin i = 0.5296(-0.0008)(+0.0011) M-circle dot orbiting at a close separation from the giant primary with a semi-major axis of a(B) = 13.56(-0.14)(+0.18) au. Conclusions. The existence of the planet HD 59686 Ab in a tight eccentric binary system severely challenges standard giant planet formation theories and requires substantial improvements to such theories in tight binaries. Otherwise, alternative planet formation scenarios such as second-generation planets or dynamical interactions in an early phase of the system's lifetime need to be seriously considered to better understand the origin of this enigmatic planet.

O tópico abordado nesta tese é a perda de massa através de ventos coronais magnetizados em estrelas de baixa massa. Ventos estelares têm sido estudados extensivamente há vários anos, tendo inicialmente como foco o vento solar. Atualmente, sabe-se que o campo magnético é essencial na aceleração e aquecimento dos ventos coronais. Apesar do conhecimento detalhado que temos da estrutura magnética do Sol, pouco se sabe sobre a configuração do campo magnético em outras estrelas. Nesta tese, é investigada a estrutura do campo magnético nas coroas de estrelas do tipo solar na Seqüência Principal e de suas predecessoras na pré Seqüência Principal através de simulações numéricas magneto-hidrodinâmicas tri-dimensionais. Aqui, consideramos de forma auto-consistente a interação entre o vento e o campo magnético e vice-versa. Dessa forma, pela interação entre forças magnéticas e forças do vento, consegue-se determinar a configuração do campo magnético e a estrutura dos ventos coronais. Realizamos um estudo de ventos de estrelas do tipo solar e a dependência dos mesmos com o parâmetro beta do plasma (a razão entre as densidades de energia térmica e magnética). Este é o primeiro estudo a realizar tal análise resolvendo as equações tri-dimensionais da magneto-hidrodinâmica ideal. Em nossas simulações, adotamos um parâmetro de aquecimento descrito por gamma, que é responsável pela aceleração térmica do vento. Então, nós analisamos ventos com intensidades de campo magnético nos pólos no intervalo de B0 = 1 a 20 G e mostramos que a estrutura do vento apresenta características que são similares à do vento coronal do Sol. No estado estacionário, a topologia do campo magnético obtida é similar para todos os casos estudados, apresentando uma configuração do tipo helmet streamer, com zonas de linhas fechadas e abertas de campo magnético co-existindo. Intensidades mais altas de campo levam a ventos mais acelerados e mais quentes. O aumento na intensidade do campo gera também uma zona morta maior no vento, i.e., os loops fechados que previnem que a matéria escape da coroa em latitudes menores que ~45 graus se estendem a maiores distâncias da estrela. Além disso, mostramos também que a força de Lorentz gera naturalmente um vento que é dependente da latitude. Ao aumentar a densidade da coroa mantendo B0 = 20 G, mostramos que o sistema volta a apresentar ventos menos acelerados e mais frios. Para um valor fixo de gamma, mostramos que o parâmetro essencial na determinação do perfil de velocidade do vento é o parâmetro beta calculado na base da coroa. Dessa forma, acredita-se que haja um grupo de ventos magnetizados que apresenta a mesma velocidade terminal independentemente das densidades de energia térmica ou magnética, desde que o parâmetro beta seja o mesmo. No entanto, essa degenerescência pode ser removida ao se comparar outros parâmetros físicos do vento, tal como a taxa de perda de massa. Nós também analisamos a influência do gamma nos nossos resultados e mostramos que ele é importante na determinação da estrutura do vento. Além disso, investigamos ventos magnetizados de estrelas de baixa massa da pré Seqüência Principal. Em particular, analisamos sob quais circunstâncias tais estrelas apresentam estruturas magnéticas alongadas (e.g., helmet streamers, proeminências do tipo slingshot, etc). Focamos especialmente em estrelas do tipo T Tauri fracas, uma vez que o tênue disco de acreção, quando presente ao redor de tais estrelas, não deve causar forte influência na estrutura do vento estelar e nem na do campo magnético coronal. Nós mostramos que o parâmetro beta do plasma é um fator decisivo na configuração do campo magnético do vento estelar. Usando parâmetros iniciais adequados ao que se é observado para tais estrelas, nós mostramos que a configuração do campo magnético pode variar entre uma configuração semelhante à de um dipolo e uma configuração com linhas fortemente colimadas em torno do eixo polar e streamers fechados ao redor do equador (configuração de multi-componentes para o campo magnético). Mostramos que as estruturas alongadas do campo magnético somente estão presentes se o parâmetro beta do plasma na base da coroa é beta0 << 1. Usando nossos modelos magneto-hidrodinâmicos, auto-consistentes, tri-dimensionais, estimamos para ventos de estrelas da pré Seqüência Principal a escala temporal de migração planetária devido a forças de arraste exercidas pelo vento em um planeta tipo hot-Jupiter (i.e., um planeta gigante que orbita muito próximo da estrela). Nosso modelo sugere que os ventos estelares de coroas com multi-componentes de campo magnético não têm influências significativas na migração de hot-Jupiters. / The subject of this thesis is the mass loss of low-mass stars through magnetized coronal winds. Stellar winds have been a topic of extensive research in Astrophysics for a long time, and their first investigations focused on the solar wind. Nowadays, we know that the magnetic field plays a crucial role in the acceleration and heating of coronal winds. Despite of the knowledge of the fine structure of the solar magnetic field, much less information is known regarding the configuration of the magnetic field in other stars. In this thesis, we investigate the structure of the magnetic field in the coronae of solar-like stars and young stars by means of three-dimensional magnetohydrodynamical numerical simulations. We self-consistently take into consideration the interaction of the outflowing wind with the magnetic field and vice versa. Hence, from the interplay between magnetic forces and wind forces, we are able to determine the configuration of the magnetic field and the structure of the coronal winds. We investigate solar-like stellar winds and their dependence on the plasma-beta parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal three-dimensional magnetohydrodynamics equations. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from B0 = 1 to 20 G and we show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. The increase of the field intensity generates a larger ``dead zone\'\' in the wind, i.e., the closed loops that inhibit matter to escape from latitudes lower than 45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B0 = 20 G, the system recovers to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We also analyze the influence of gamma in our results and we show that it is also important in determining the wind structure. We further investigate magnetized stellar winds of low-mass pre-main-sequence stars. In particular we analyze under which circumstances these stars present elongated magnetic features (e.g., helmet streamers, slingshot prominences, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind neither on the coronal magnetic field. We show that the plasma-beta parameter is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multicomponent configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta0 << 1. Using our self-consistent three-dimensional magnetohydrodynamical model, we estimate for the stellar winds of pre-main-sequence stars the timescale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter (i.e., on a giant planet that orbits very close to the star). Our model suggests that the stellar wind of these multicomponent coronae are not expected to have significant influence on the migration of hot-Jupiters.

O tópico abordado nesta tese é a perda de massa através de ventos coronais magnetizados em estrelas de baixa massa. Ventos estelares têm sido estudados extensivamente há vários anos, tendo inicialmente como foco o vento solar. Atualmente, sabe-se que o campo magnético é essencial na aceleração e aquecimento dos ventos coronais. Apesar do conhecimento detalhado que temos da estrutura magnética do Sol, pouco se sabe sobre a configuração do campo magnético em outras estrelas. Nesta tese, é investigada a estrutura do campo magnético nas coroas de estrelas do tipo solar na Seqüência Principal e de suas predecessoras na pré Seqüência Principal através de simulações numéricas magneto-hidrodinâmicas tri-dimensionais. Aqui, consideramos de forma auto-consistente a interação entre o vento e o campo magnético e vice-versa. Dessa forma, pela interação entre forças magnéticas e forças do vento, consegue-se determinar a configuração do campo magnético e a estrutura dos ventos coronais. Realizamos um estudo de ventos de estrelas do tipo solar e a dependência dos mesmos com o parâmetro beta do plasma (a razão entre as densidades de energia térmica e magnética). Este é o primeiro estudo a realizar tal análise resolvendo as equações tri-dimensionais da magneto-hidrodinâmica ideal. Em nossas simulações, adotamos um parâmetro de aquecimento descrito por gamma, que é responsável pela aceleração térmica do vento. Então, nós analisamos ventos com intensidades de campo magnético nos pólos no intervalo de B0 = 1 a 20 G e mostramos que a estrutura do vento apresenta características que são similares à do vento coronal do Sol. No estado estacionário, a topologia do campo magnético obtida é similar para todos os casos estudados, apresentando uma configuração do tipo helmet streamer, com zonas de linhas fechadas e abertas de campo magnético co-existindo. Intensidades mais altas de campo levam a ventos mais acelerados e mais quentes. O aumento na intensidade do campo gera também uma zona morta maior no vento, i.e., os loops fechados que previnem que a matéria escape da coroa em latitudes menores que ~45 graus se estendem a maiores distâncias da estrela. Além disso, mostramos também que a força de Lorentz gera naturalmente um vento que é dependente da latitude. Ao aumentar a densidade da coroa mantendo B0 = 20 G, mostramos que o sistema volta a apresentar ventos menos acelerados e mais frios. Para um valor fixo de gamma, mostramos que o parâmetro essencial na determinação do perfil de velocidade do vento é o parâmetro beta calculado na base da coroa. Dessa forma, acredita-se que haja um grupo de ventos magnetizados que apresenta a mesma velocidade terminal independentemente das densidades de energia térmica ou magnética, desde que o parâmetro beta seja o mesmo. No entanto, essa degenerescência pode ser removida ao se comparar outros parâmetros físicos do vento, tal como a taxa de perda de massa. Nós também analisamos a influência do gamma nos nossos resultados e mostramos que ele é importante na determinação da estrutura do vento. Além disso, investigamos ventos magnetizados de estrelas de baixa massa da pré Seqüência Principal. Em particular, analisamos sob quais circunstâncias tais estrelas apresentam estruturas magnéticas alongadas (e.g., helmet streamers, proeminências do tipo slingshot, etc). Focamos especialmente em estrelas do tipo T Tauri fracas, uma vez que o tênue disco de acreção, quando presente ao redor de tais estrelas, não deve causar forte influência na estrutura do vento estelar e nem na do campo magnético coronal. Nós mostramos que o parâmetro beta do plasma é um fator decisivo na configuração do campo magnético do vento estelar. Usando parâmetros iniciais adequados ao que se é observado para tais estrelas, nós mostramos que a configuração do campo magnético pode variar entre uma configuração semelhante à de um dipolo e uma configuração com linhas fortemente colimadas em torno do eixo polar e streamers fechados ao redor do equador (configuração de multi-componentes para o campo magnético). Mostramos que as estruturas alongadas do campo magnético somente estão presentes se o parâmetro beta do plasma na base da coroa é beta0 << 1. Usando nossos modelos magneto-hidrodinâmicos, auto-consistentes, tri-dimensionais, estimamos para ventos de estrelas da pré Seqüência Principal a escala temporal de migração planetária devido a forças de arraste exercidas pelo vento em um planeta tipo hot-Jupiter (i.e., um planeta gigante que orbita muito próximo da estrela). Nosso modelo sugere que os ventos estelares de coroas com multi-componentes de campo magnético não têm influências significativas na migração de hot-Jupiters. / The subject of this thesis is the mass loss of low-mass stars through magnetized coronal winds. Stellar winds have been a topic of extensive research in Astrophysics for a long time, and their first investigations focused on the solar wind. Nowadays, we know that the magnetic field plays a crucial role in the acceleration and heating of coronal winds. Despite of the knowledge of the fine structure of the solar magnetic field, much less information is known regarding the configuration of the magnetic field in other stars. In this thesis, we investigate the structure of the magnetic field in the coronae of solar-like stars and young stars by means of three-dimensional magnetohydrodynamical numerical simulations. We self-consistently take into consideration the interaction of the outflowing wind with the magnetic field and vice versa. Hence, from the interplay between magnetic forces and wind forces, we are able to determine the configuration of the magnetic field and the structure of the coronal winds. We investigate solar-like stellar winds and their dependence on the plasma-beta parameter (the ratio between thermal and magnetic energy densities). This is the first study to perform such analysis solving the fully ideal three-dimensional magnetohydrodynamics equations. We adopt in our simulations a heating parameter described by gamma, which is responsible for the thermal acceleration of the wind. We analyze winds with polar magnetic field intensities ranging from B0 = 1 to 20 G and we show that the wind structure presents characteristics that are similar to the solar coronal wind. The steady-state magnetic field topology for all cases is similar, presenting a configuration of helmet streamer-type, with zones of closed field lines and open field lines coexisting. Higher magnetic field intensities lead to faster and hotter winds. The increase of the field intensity generates a larger ``dead zone\'\' in the wind, i.e., the closed loops that inhibit matter to escape from latitudes lower than 45 degrees extend farther away from the star. The Lorentz force leads naturally to a latitude-dependent wind. We show that by increasing the density and maintaining B0 = 20 G, the system recovers to slower and cooler winds. For a fixed gamma, we show that the key parameter in determining the wind velocity profile is the beta-parameter at the coronal base. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-beta parameter is the same. This degeneracy, however, can be removed if we compare other physical parameters of the wind, such as the mass-loss rate. We also analyze the influence of gamma in our results and we show that it is also important in determining the wind structure. We further investigate magnetized stellar winds of low-mass pre-main-sequence stars. In particular we analyze under which circumstances these stars present elongated magnetic features (e.g., helmet streamers, slingshot prominences, etc). We focus on weak-lined T Tauri stars, as the presence of the tenuous accretion disk is not expected to have strong influence on the structure of the stellar wind neither on the coronal magnetic field. We show that the plasma-beta parameter is a decisive factor in defining the magnetic configuration of the stellar wind. Using initial parameters within the observed range for these stars, we show that the coronal magnetic field configuration can vary between a dipole-like configuration and a configuration with strong collimated polar lines and closed streamers at the equator (multicomponent configuration for the magnetic field). We show that elongated magnetic features will only be present if the plasma-beta parameter at the coronal base is beta0 << 1. Using our self-consistent three-dimensional magnetohydrodynamical model, we estimate for the stellar winds of pre-main-sequence stars the timescale of planet migration due to drag forces exerted by the stellar wind on a hot-Jupiter (i.e., on a giant planet that orbits very close to the star). Our model suggests that the stellar wind of these multicomponent coronae are not expected to have significant influence on the migration of hot-Jupiters.