A principal scientific goal of the Gemini Planet Imager (GPI) is obtaining milliarcsecond astrometry to constrain exoplanet orbits. However, astrometry of directly imaged exoplanets is subject to biases, systematic errors, and speckle noise. Here, we describe an analytical procedure to forward model the signal of an exoplanet that accounts for both the observing strategy (angular and spectral differential imaging) and the data reduction method (Karhunen-Loeve Image Projection algorithm). We use this forward model to measure the position of an exoplanet in a Bayesian framework employing Gaussian processes and Markov-chain Monte Carlo to account for correlated noise. In the case of GPI data on beta Pic b, this technique, which we call Bayesian KLIP-FM Astrometry (BKA), outperforms previous techniques and yields 1 sigma errors at or below the one milliarcsecond level. We validate BKA by fitting a Keplerian orbit to 12 GPI observations along with previous astrometry from other instruments. The statistical properties of the residuals confirm that BKA is accurate and correctly estimates astrometric errors. Our constraints on the orbit of beta Pic b firmly rule out the possibility of a transit of the planet at 10-sigma significance. However, we confirm that the Hill sphere of beta Pic b will transit, giving us a rare chance to probe the circumplanetary environment of a young, evolving exoplanet. We provide an ephemeris for photometric monitoring of the Hill sphere transit event, which will begin at the start of April in 2017 and finish at the end of January in 2018.

We present ALMA observations of the 2M1207 system, a young binary made of a brown dwarf with a planetary-mass companion at a projected separation of about 40 au. We detect emission from dust continuum at 0.89 mm and from the J = 3 - 2 rotational transition of CO from a very compact disk around the young brown dwarf. The small radius found for this brown dwarf disk may be due to truncation from the tidal interaction with the planetary-mass companion. Under the assumption of optically thin dust emission, we estimate. a dust mass of 0.1 M-circle plus. for the 2M1207A disk and a 3 sigma upper limit of similar to 1 M-Moon for dust surrounding 2M1207b, which is the tightest upper limit obtained so far for the mass of dust particles surrounding a young planetary-mass companion. We discuss the impact of this and other non-detections of young planetary-mass companions for models of planet formation that predict circumplanetary material to surround these objects.

The ice-rich crust of dwarf planet 1 Ceres is the source of a tenuous water exosphere, and the behavior of this putative exosphere is investigated with model calculations. Outgassing water molecules seasonally condense around the winter pole in an optically thin layer. This seasonal cap reaches an estimated mass of at least 2 x 10(3) kg, and the aphelion summer pole may even retain water throughout summer. If this reservoir is suddenly released by a solar energetic particle event, it would form a denser transient water exosphere. Our model calculations also explore species other than H2O. Light exospheric species escape rapidly from Ceres due to its low gravity, and hence their exospheres dissipate soon after their respective source has faded. For example, the theoretical turn-over time in a water exosphere is only 7 hr. A significant fraction of CO2 and SO2 molecules can get trapped and stored in perennially shadowed regions at the current spin axis orientation, but not at the higher spin axis tilt, leaving H2O as the only common volatile expected to accumulate in polar cold traps over long timescales. The D/H fractionation during migration to the cold traps is only about 10%.

Since the first discovery of an extrasolar planet around a solar-type star, observers have detected over 500 planets outside the solar system. Many of these planets have Jovian masses and orbit their host stars in orbits of only a few days, the so-called 'Hot Jupiters'. At such close proximity to their parent stars, strong tidal interactions between the two bodies are expected to cause significant secular spin-orbit evolution. This thesis tackles two problems regarding the tidal evolution of short-period extrasolar planets. In the first part, we adopt a simple model of the orbit-averaged effects of tidal friction, to study the tidal evolution of planets on inclined orbits. We also analyse the effects of stellar magnetic braking. We then discuss the implications of our results for the importance of Rossiter-Mclaughlin effect observations. In the second part, we study the mechanisms of tidal dissipation in solar-type stars. In particular, internal gravity waves are launched at the interface of the convection and radiation zones of such a star, by the tidal forcing of a short-period planet. The fate of these waves as they approach the centre of the star is studied, primarily using numerical simulations, in both two and three dimensions. We find that the waves undergo instability and break above a critical amplitude. A model for the tidal dissipation that results from this process is presented, and its validity is verified by numerical integrations of the linear tidal response, in an extensive set of stellar models. The dissipation is efficient, and varies by less than an order of magnitude between all solar-type stars, throughout their main-sequence lifetimes, for a given planetary orbit. The implications of this mechanism for the survival of short-period extrasolar planets is discussed, and we propose a possible explanation for the survival of all of the extrasolar planets currently observed in short-period orbits around F, G and K stars. We then perform a stability analysis of a standing internal gravity wave near the centre of a solar-type star, to understand the early stages of the wave breaking process in more detail, and to determine whether the waves are subject to weaker parametric instabilities, below the critical amplitude required for wave breaking. We discuss the relevance of our results to our explanation for the survival of short-period planets presented in the second part of this thesis. Finally, we propose an alternative mechanism of tidal dissipation, involving the gradual radiative damping of the waves. Based on a simple estimate, it appears that this occurs even for low mass planets. However, it is in conflict with current observations since it would threaten the survival of all planets in orbits shorter than 2 days. We discuss some hydrodynamic instabilities and magnetic stresses which may prevent this process.

Orientador: Othon Cabo Winter / Coorientadora: Carla de Souza Torres / Banca: André Izidoro Ferreira da Costa / Banca: Fernando Virgilio Roig / Resumo: Estudos de formação planetária são normalmente baseados em simulações numéricas de N-corpos, onde as colisões envolvendo planetesimais e embriões são geralmente consideradas construtivas, ou seja, geram um novo corpo cuja massa é a soma das massas dos planetas envolvidos e o momento linear total é conservado. Essa aproximação funciona razoavelmente bem para a formação da Terra, Vênus e Marte. Entretanto, o planeta Mercúrio apresenta características que o classificam como sendo um corpo formado basicamente de núcleo, devido à estreita camada de manto. Isso supostamente seria consequência de acreções ineficientes, em que dois corpos em estágio avançado de formação (protoplanetas) colidem e resultam em dois outros corpos com parte da matéria podendo ser perdida. Este tipo de informação pode ser obtida a partir de simulações numéricas em que sejam registradas as condições de colisão. Aqui, nós utilizamos parâmetros para nos ajudar a analisar as colisões, a eterminar colisões de acreção ineficiente e, posteriormente analisar se estas colisões levar a condição da formação de um planeta como Mercúrio de acordo com os cenários propostos por alguns trabalhos, tais como Asphaug & Reufer (2014). Nesse sentido, realizamos simulações baseadas nos modelos de simulações numéricas de N-corpos conforme Izidoro et al. (2014), analisamos as simulações numéricas de Izidoro et al. (2015) e reproduzimos os mapas dos resultados de colisão a partir das leis de escala determinadas por Leinhardt & Ste... (Resumo completo, clicar acesso eletrônico abaixo) / Mestre

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<p> Fragmentation during encounters between protostellar disks provides a possible scenario for the formation of substellar objects such as brown dwarfs and planets. A series of simulations of protostellar disk encounters were performed to investigate the fragmentation under different encounter parameters, and to characterize the properties of any resultant fragments. It was found that the initial disk minimum Toomre Q must satisfy Qini ;S 1.1 for the fragmentation to be induced by the encounters. Fragments of substellar mass can form via disk fragmentation, shock layer fragmentation and tidal tail fragmentation, and the effectiveness of each mechanism is closely related to the initial disk configuration. The fragmentation is also constrained by the relative encounter velocity since the number of fragments decreases quickly with increasing velocity. </p> <p> In comparing to previous studies of protostellar disk encounters it was also found that resolving both the local Jeans Mass during the encounter and the disks' vertical structure are critical to prevent artificial fragmentation and give the correct picture. Heating and cooling rates were estimated in both the optically thin and thick regimes. The comparison between the two indicates that during strong impacts the heating rate increases rapidly but is still comparable to the cooling rate, so the locally isothermal equation of state used in this study is an acceptable approximation. </p> <p> 32 clumps formed in various Qini = 0.9 disk-disk encounters were taken as the sample in an analysis of fragment properties and prospects for their further evolution. The results show that the clump masses are all less than the hydrogen burning mass limit ~ 0.075M0 , so the objects are substellar. Most of the clumps are of brown dwarf mass since the formation of planetary mass clumps is suppressed due to numerical resolution. The mass distribution is broadly consistent to the observed initial mass function in Pleiades. The clumps have highly flattened disk-like shapes and possess large spin angular momentum, which implies that young brown dwarfs may develop disks, jets, or planetary mass companions. About one third of the fragments are unbound to the stars and likely to form free floating brown dwarfs. Orbital analyses of the clumps which are bound to the stars show that there is a lack of close brown dwarf companions ( R < 3 AU), which is consistent to the observed "brown dwarf desert". Many of the orbits are highly eccentric and intersect with other orbits, so ejection of some clumps due to gravitational scattering is likely. Also, dispersion of gas during the encounter and the high spin angular momentum of the clumps may provide mechanisms other than ejection to prevent the clumps from accreting more mass, making the simulated clumps representative of the long term substellar mass function. </p> / Thesis / Master of Science (MSc)

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 140-143. / Ph.D.