We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to millimeter-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to-gas ratio intersect the allowed region for streaming instability-induced gravitational collapse. Using an approximate analytic treatment, we first show that drifting particles define a track in metallicity-stopping time space whose only substantial dependence is on the disk's angular momentum transport efficiency. Prompt planetesimal formation is feasible for high particle accretion rates (relative to the gas, (M) over dot(p)/(M) over dot greater than or similar to 3 x 10(-2) for alpha = 10(-2)), which could only be sustained for a limited period of time. If it is possible, it would lead to the deposition of a broad and massive belt of planetesimals with a sharp outer edge. Numerically including turbulent diffusion and vapor condensation processes, we find that a modest enhancement of solids near the snow line occurs for centimeter-sized particles, but that this is largely immaterial for planetesimal formation. We note that radial drift couples planetesimal formation across radii in the disk, and suggest that considerations of planetesimal formation favor a model in which the initial deposition of material for giant planet cores occurs well beyond the snow line.

We present an outline of basic assumptions and governing structural equations describing atmospheres of sub-stellar mass objects, in particular the extrasolar giant planets and brown dwarfs. Although most of the presentation of the physical and numerical background is generic, details of the implementation pertain mostly to the code COOLTLUSTY. We also present a review of numerical approaches and computer codes devised to solve the structural equations, and make a critical evaluation of their efficiency and accuracy.

Abstract The present thesis investigates the dynamics of wave structures in dense planetary rings by employing hydrodynamic models, along with local N-body simulations of the particulate ring flow. The focus is on the large-scale satellite induced spiral density waves as well as on the free short-scale waves generated by the viscous overstability in Saturn's A and B rings. An analytic weakly nonlinear model is derived by using perturbation theory based on multi-scale methods to compute the damping behavior of nonlinear spiral density waves in a planetary ring subject to viscous overstability. In order to study the complex spatio-temporal evolution of these wave structures, numerical schemes are developed to integrate the hydrodynamical equations in time on large radial domains, taking into account collective self-gravity forces of the ring material, as well as the forcing by an external satellite. The required numerical stability and accuracy is achieved by applying Flux-Vector-Splitting methods aligned with advanced shock-capturing techniques. The free short-scale overstable waves are also investigated with local N-body simulations of the sheared ring flow. In particular, the influence of collective self-gravity between the ring particles as well as the periodic forcing due to a nearby Lindblad resonance on the overstable wave pattern is considered. The linear stability criterion for spiral density waves in Saturn’s rings is found to be identical to the condition for the onset of spontaneous viscous overstability in the limit of long wavelengths and agrees with the stability criterion for density waves derived by Borderies et al. within the streamline formalism. The derived nonlinear damping behavior of density waves can be very different from what has previously been thought. The role of collective self-gravity on the nonlinear evolution of short-scale overstable waves is determined, reconciling the partly contradicting results of previous studies. It is shown that collective self-gravity plays an important role in setting the length-scale on which the nonlinear overstable waves saturate in a planetary ring. A co-existence of spiral density waves and short-scale overstable waves is modeled in terms of one-dimensional large-scale hydrodynamical integrations. Due to the restriction to one space dimension, certain terms in the hydrodynamical equations that arise from the spiral shape of a density wave need to be approximated based on the weakly nonlinear model. These integrations reveal that density waves and spontaneous viscous overstability undergo complex interactions. In particular it is found that, depending on the relative magnitude of the two wave structures, the presence of short-scale overstable waves can lead to a damping of an overstable density wave and vice versa, density waves can suppress overstability. The effect of a density wave on the viscous overstability is also studied in terms of a simplified axisymmetric model of a ring perturbed by a nearby Lindblad resonance. A linear hydrodynamic stability analysis and local N-body simulations of this model system conform the corresponding results of the large-scale hydrodynamical integrations.

We demonstrate a path to hitherto unachievable differential photometric precisions from the ground, both in the optical and near-infrared (NIR), using custom-fabricated beam-shaping diffusers produced using specialized nanofabrication techniques. Such diffusers mold the focal plane image of a star into a broad and stable top-hat shape, minimizing photometric errors due to non-uniform pixel response, atmospheric seeing effects, imperfect guiding, and telescope-induced variable aberrations seen in defocusing. This PSF reshaping significantly increases the achievable dynamic range of our observations, increasing our observing efficiency and thus better averages over scintillation. Diffusers work in both collimated and converging beams. We present diffuser-assisted optical observations demonstrating 62(-16)(+26) ppm precision in 30 minute bins on a nearby bright star 16 Cygni A (V = 5.95) using the ARC 3.5 m telescope-within a factor of similar to 2 of Kepler's photometric precision on the same star. We also show a transit of WASP-85-Ab (V = 11.2) and TRES-3b (V = 12.4), where the residuals bin down to 180(-41)(+66) ppm in 30 minute bins for WASP-85-Ab-a factor of similar to 4 of the precision achieved by the K2 mission on this target-and to 101 ppm for TRES-3b. In the NIR, where diffusers may provide even more significant improvements over the current state of the art, our preliminary tests demonstrated 137(-36)(+64) ppm precision for a K-S = 10.8 star on the 200 inch. Hale Telescope. These photometric precisions match or surpass the expected photometric precisions of TESS for the same magnitude range. This technology is inexpensive, scalable, easily adaptable, and can have an important and immediate impact on the observations of transits and secondary eclipses of exoplanets.

We report the results of high-angular-resolution observations that search for exozodiacal light in a sample of main sequence stars and sub-giants. Using the "jouvence" of the fiber linked unit for optical recombination (JouFLU) at the center for high angular resolution astronomy (CHARA) telescope array, we have observed a total of 44 stars. Out of the 44 stars, 33 are new stars added to the initial, previously published survey of 42 stars performed at CHARA with the fiber linked unit for optical recombination (FLUOR). Since the start of the survey extension, we have detected a K-band circumstellar excess for six new stars at the similar to 1% level or higher, four of which are known or candidate binaries, and two for which the excess could be attributed to exozodiacal dust. We have also performed follow-up observations of 11 of the stars observed in the previously published survey and found generally consistent results. We do however detect a significantly larger excess on three of these follow-up targets: Altair, v And and kappa CrB. Interestingly, the last two are known exoplanet host stars. We perform a statistical analysis of the JouFLU and FLUOR samples combined, which yields an overall exozodi detection rate of 21.7(-4.1)(+5.7) %. We also find that the K-band excess in FGK-type stars correlates with the existence of an outer reservoir of cold (less than or similar to 100 K) dust at the 99% confidence level, while the same cannot be said for A-type stars.

In the last decade, over a million stars were monitored to detect transiting planets. Manual interpretation of potential exoplanet candidates is labour intensive and subject to human error, the results of which are difficult to quantify. Here we present a new method of detecting exoplanet candidates in large planetary search projects that, unlike current methods, uses a neural network. Neural networks, also called 'deep learning' or 'deep nets', are designed to give a computer perception into a specific problem by training it to recognize patterns. Unlike past transit detection algorithms, deep nets learn to recognize planet features instead of relying on hand-coded metrics that humans perceive as the most representative. Our convolutional neural network is capable of detecting Earth-like exoplanets in noisy time series data with a greater accuracy than a least-squares method. Deep nets are highly generalizable allowing data to be evaluated from different time series after interpolation without compromising performance. As validated by our deep net analysis of Kepler light curves, we detect periodic transits consistent with the true period without any model fitting. Our study indicates that machine learning will facilitate the characterization of exoplanets in future analysis of large astronomy data sets.

We present an optical eclipse observation of the hot Jupiter WASP-12b using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. These spectra allow us to place an upper limit of A(g) < 0.064 (97.5% confidence level) on the planet's white light geometric albedo across 290-570 nm. Using six wavelength bins across the same wavelength range also produces stringent limits on the geometric albedo for all bins. However, our uncertainties in eclipse depth are similar to 40% greater than the Poisson limit and may be limited by the intrinsic variability of the Sun-like host star-the solar luminosity is known to vary at the 10(-4) level on a timescale of minutes. We use our eclipse depth limits to test two previously suggested atmospheric models for this planet: Mie scattering from an aluminum-oxide haze or cloud-free Rayleigh scattering. Our stringent nondetection rules out both models and is consistent with thermal emission plus weak Rayleigh scattering from atomic hydrogen and helium. Our results are in stark contrast with those for the much cooler HD 189733b, the only other hot Jupiter with spectrally resolved reflected light observations; those data showed an increase in albedo with decreasing wavelength. The fact that the first two exoplanets with optical albedo spectra exhibit significant differences demonstrates the importance of spectrally resolved reflected light observations and highlights the great diversity among hot Jupiters.

Disks around brown dwarfs (BDs) are excellent laboratories to study the first steps of planet formation in cold and low-mass disk conditions. The radial-drift velocities of dust particles in BD disks higher than in disks around more massive stars. Therefore, BD disks are expected to be more depleted in millimeter-sized grains compared to disks around T Tauri or Herbig Ae/Be stars. However, recent millimeter observations of BD disks revealed low millimeter spectral indices, indicating the presence of large grains in these disks and challenging models of dust evolution. We present 3 mm photometric observations carried out with the IRAM/Plateau de Bure Interferometer (PdBI) of three BD disks in the Taurus star-forming region, which have been observed with ALMA at 0.89 mm. The disks were not resolved and only one was detected with enough confidence (similar to 3.5 sigma) with PdBI. Based on these observations, we obtain the values and lower limits of the spectral index and find low values (alpha(mm) less than or similar to 3.0). We compare these observations in the context of particle trapping by an embedded planet, a promising mechanism to explain the observational signatures in more massive and warmer disks. We find, however, that this model cannot reproduce the current millimeter observations for BD disks, and multiple-strong pressure bumps globally distributed in the disk remain as a favorable scenario to explain observations. Alternative possibilities are that the gas masses in the BD disk are very low (similar to 2 x 10(-3) M-Jup) such that the millimeter grains are decoupled and do not drift, or fast growth of fluffy aggregates.

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.