Recent studies suggest that many protoplanetary disks around pre-main sequence stars with inferred ages of 1-5 Myr (known as Class II protostars) may contain insufficient mass to form giant planets. This may be because by this stage much of the material in the disk has already grown into larger bodies, hiding the material from sight. If this is the case, then these older disks may not be an accurate representation of the initial mass budget in disks for forming planets. To test this hypothesis, I have observed a sample of protostars in the Taurus star forming regions identified as Class I in multiple independent surveys, whose young (<1 Myr old) disks are more likely to represent the initial mass budget of protoplanetary disks. For my dissertation I have used detailed radiative transfer modeling of a multi-wavelength dataset to determine the geometry of the circumstellar material and measure the mass of the disks around these protostars. I discuss how the inferred disk mass distribution for this sample compares with results for the existing 1-5 Myr old disk samples, and what these results imply for giant planet formation. Next, I discuss the cases of three separate, individual Class I protostars discovered through my ongoing survey of Class I protostars whose disks are all of particular interest, each for its own reasons. Each of these disks may provide clues that even at the young ages of Class I protostars, planet formation may already be well underway in their disks. Finally, large disk mass surveys of large star forming regions like the Orion Nebula Cluster may be contaminated by free-free emission from disks that are being photoevaporated by nearby massive stars. I discuss my work with the VLA to constrain the free-free emission spectra for these sources so that current and future millimeter surveys can accurately measure disk masses in the ONC.

Recent high angular resolution observations of protoplanetary disks at different wavelengths have revealed several kinds of structures, including multiple bright and dark rings. Embedded planets are the most used explanation for such structures, but there are alternative models capable of shaping the dust in rings as it has been observed. We assume a disk around a Herbig star and investigate the effect that ice lines have on the dust evolution, following the growth, fragmentation, and dynamics of multiple dust size particles, covering from 1 mu m to 2 m sized objects. We use simplified prescriptions of the fragmentation velocity threshold, which is assumed to change radially at the location of one, two, or three ice lines. We assume changes at the radial location of main volatiles, specifically H2O, CO2, and NH3. Radiative transfer calculations are done using the resulting dust density distributions in order to compare with current multiwavelength observations. We find that the structures in the dust density profiles and radial intensities at different wavelengths strongly depend on the disk viscosity. A clear gap of emission can be formed between ice lines and be surrounded by ring-like structures, in particular between the H2O and CO2 (or CO). The gaps are expected to be shallower and narrower at millimeter emission than at near-infrared, opposite to model predictions of particle trapping. In our models, the total gas surface density is not expected to show strong variations, in contrast to other gap-forming scenarios such as embedded giant planets or radial variations of the disk viscosity.

No description available.

<p> Planetary migration has become one of the most important processes in planet formation since the first discovery of an exoplanet around 51Peg. A decade after the discovery, the total number of exoplanets has increased to about three hundred. Theoretical work has shown that the disk configuration in which planets are formed strongly affects the subsequent migration of planets within them. Disks evolve and their shape transits from flared to fiat. This is thought to arise because of dust settling. We take this effect into account in our models of planet migration in protoplanetary disks that are heated by the radiation of their central stars. In particular we solve the radiative transfer equation for disks by means of the Monte Carlo method, and then consider planetary migration. We focus on planets around very low mass stars (VLMSs). </p> <p> Our calculations reproduce the disk configurations of Chiang & Goldreich (1997). As dust settles, the superheated and inner layer declines toward the mid-plane. At the same time, dust settling causes the temperature of the upper layer to increase and that of the inner layer to decrease. In order to calculate the migration time accurately, we include the gravity of planets, which causes the density around them to be compressed. This results in shadowing (in front of the planet) and illumination (behind the planet) regions. We included disk evolution by taking into account the effect of dust settling. We found that dust settling itself (without planetary gravity) can reduce the migration time by a factor of 8. When we included the gravity of planets, the effect of dust settling is somewhat washed out. This is because the effect of dust settling on migration acts in a similar way to that of planetary gravity. Thus, when the migration time without dust settling is compared to the case of dust settling (including planetary gravity), dust settling can reduce the migration time by a factor of 2. </p> <p> We also found that the migration time of massive planets(> 5MEB) in such low mass disks, for both cases, is comparable to the disk life time ( rv 107 years). This suggests that planets around VLMS do not plunge into the star within a disk lifetime. This finding is consistent with the discovery of the super-Earth (rv 5.5MEB) at 2.6 AU around M dwarf (Beaulieu et al., 2006). For lower mass planets, the migration time is about two orders of magnitude longer than the disk life time. Thus, the long planetary migration around VLMS does not cause any serious time mismatch problem as in the case of classical T Tauri star system. </p> / Thesis / Master of Science (MSc)

A long-standing question in planet formation is the origin of planetesimals, the kilometre-sized precursors to protoplanets. Asteroids and distant Kuiper Belt objects are believed to be remnant planetesimals from the beginnings of our Solar system. A leading mechanism for explaining the formation of these bodies directly from centimetre-sized dust pebbles is the streaming instability (SI). Using high resolution numerical simulations of protoplanetary discs, we probe the behavior of the non-linear SI and planetesimal formation in previously unexplored configurations. Small variations in initial state of the disc can lead to different macroscopic outcomes such as the total mass converted to planetesimals, or the distribution of planetesimal masses. These properties can vary considerably within large simulations, or across smaller simulations re-run with different initial perturbations. However, there is a similar spread in outcomes between multiple smaller simulations and between smaller sub-regions in larger simulations. In small simulations, filaments preferentially form rings while in larger simulations they are truncated. Larger domains permit dynamics on length scales inaccessible to the smaller domains. However, the overall mass concentrated in filaments across various length scales is consistent in all simulations. Small simulations in our suite struggle to resolve dynamics at the natural filament separation length scale, which restricts the possible filament configurations in these simulations. We also model discs with multiple grain species, sampling a size distribution predicted from theories of grain coagulation and fragmentation. The smallest grains do not participate in the formation of planetesimals or filaments, even while they co-exist with dust that readily forms such dense features. For both single-grain and multiple-grain models, we show that the clumping of dust into dense features results in saturated thermal emission, requiring an observational mass correction factor that can be as large as 20-80\%. Finally, we present preliminary work showing that the critical dust-to-gas mass ratio required to trigger the SI can vary between 3D and 2D simulations. / Thesis / Doctor of Philosophy (PhD)

The research presented in this thesis investigates several aspects of the evolutionary processes of T Tauri stars and their accompanying circumstellar disks. The versatile Monte Carlo radiation transfer technique, with several modifications and extensions, is used throughout to study the structure and constitution of both the circumstellar disk at large and the changeable and dynamic inner disk regions. The photopolarimetric variability of AA Tau in the Taurus star forming region is modelled in a fully 3D manner. I find that a magnetospherically induced warp in the accretion disk at roughly the stellar co-rotation radius occults the star and reproduces both the observed period and duration and the required brightness and polarisation variations. The model SEDs allow estimates of the disk mass, radial extent and large- scale density structure. Using a modified SPH code we find the interaction of a 5.2kG stellar magnetic field inclined at 30° to the rotation axis with the disk, is capable of generating a warp of the size and shape needed to reproduce the observed variations. Modified Monte Carlo models capable of incorporating any number of dust particle grain sizes distributed throughout the disk in vertical and radial distributions, in a fully 3D manner are presented. This versatile tool allows the investigation of evolutionary processes such as dust settling and grain growth predicted to occur in T Tauri sources as they age. A Mie Scattering code was also adapted and incorporated into the models allowing us to determine optical properties for dust grains and distributions of any size. I present model SEDs fitting the latest publicly available IR data for a number of T Tauri sources and reproduce the observational effects of dust grain growth and settling with a high degree of success. The fits are by no means unique and the structural parameters required to produce them are quite uncertain but it is possible to determine useful information on the larger scale structure and bulk constituents of these disks. A fully 3D non-LTE radiative transfer code using CO line emissions as a tracer of the disk dynamics and able to simulate any disk structure or geometry, either analytical or imported from a hydrodynamic simulation, is presented. Signatures attributed to the disk dynamics and spiral density structure derived from hydrodynamic simulations of massive disks are investigated and resolved. Line profiles and contour maps of the velocity of the emitting material are generated and compared with observations.

Protoplanetary discs set the initial conditions for planet formation. By combining observations with detailed modelling, it is possible to constrain the physics and chemistry in such discs. I have used the detailed thermo-chemical disc model ProDiMo to explore the characteristics of the gas in protoplanetary discs, particularly in Herbig Ae objects. I have assessed the ability of various observational data to trace the disc properties. This has involved a number of different approaches. Firstly I compute a series of disc models with increasing mass, in order to test the diagnostic powers of various emission lines, in particular as gas mass tracers. This approach is then expanded to a large multiparameter grid of ~ 10 5 disc models. I have helped to develop a tool for analysing and plotting the huge quantity of data presented by such a model grid. Following this approach I move on to a detailed study of the Herbig Ae star HD 163296, attempting to fit the large wealth of available observations simultaneously. These include new Herschel observations of the far-infrared emission lines, as well as interferometric CO observations and a large number of continuum data. This study addresses the topical issues of the disc gas/dust ratio, and the treatment of the disc outer edge. It explores the effects of dust settling, UV variability and stellar X-ray emission on the disc chemistry and line emission. There is possible evidence for gas-depletion in the disc of HD 163296, with the line emission enhanced by dust settling, which would indicate a later evolutionary stage for this disc than suggested by other studies. Finally, I work to improve the treatment of the gas heating/cooling balance in ProDiMo, by introducing a non-LTE treatment of the atomic hydrogen line transitions and bound-free continuum transitions. I explore the effects of this on the disc chemical and thermal structure, and assess its impact in terms of the observable quantities.

We derived 90% confidence limits (CLs) on the interstellar number density (rho(CL)(IS)) of interstellar objects (ISOs; comets and asteroids) as a function of the slope of their size-frequency distribution (SFD) and limiting absolute magnitude. To account for gravitational focusing, we first generated a quasi-realistic ISO population to similar to 750 au from the Sun and propagated it forward in time to generate a steady state population of ISOs with heliocentric distance <50 au. We then simulated the detection of the synthetic ISOs using pointing data for each image and average detection efficiencies for each of three contemporary solar system surveys-Pan-STARRS1, the Mt. Lemmon Survey, and the Catalina Sky Survey. These simulations allowed us to determine the surveys' combined ISO detection efficiency under several different but realistic modes of identifying ISOs in the survey data. Some of the synthetic detected ISOs had eccentricities as small as 1.01, which is in the range of the largest eccentricities of several known comets. Our best CL of rho(CL)(SI) = 1.4 x 10(-4) au(-3) implies that the expectation that extra-solar systems form like our solar system, eject planetesimals in the same way, and then distribute them throughout the Galaxy, is too simplistic, or that the SFD or behavior of ISOs as they pass through our solar system is far from expectation.

The presence of a giant planet in a low-viscosity disc can create a gap edge in the disc's radial density profile sharp enough to excite the Rossby wave instability. This instability may evolve into dust-trapping vortices that might explain the `banana-shaped' features in recently observed asymmetric transition discs with inner cavities. Previous hydrodynamical simulations of planet-induced vortices have neglected the time-scale of hundreds to thousands of orbits to grow a massive planet to Jupiter size. In this work, we study the effect of a giant planet's runaway growth time-scale on the lifetime and characteristics of the resulting vortex. For two different planet masses (1 and 5 Jupiter masses) and two different disc viscosities (alpha= 3 x 10-4 and 3 x 10-5), we compare the vortices induced by planets with several different growth time-scales between 10 and 4000 planet orbits. In general, we find that slowly-growing planets create significantly weaker vortices with lifetimes and surface densities reduced by more than 50 per cent. For the higher disc viscosity, the longest growth time-scales in our study inhibit vortex formation altogether. Additionally, slowly-growing planets produce vortices that are up to twice as elongated, with azimuthal extents well above 180. in some cases. These unique, elongated vortices likely create a distinct signature in the dust observations that differentiates them from the more concentrated vortices that correspond to planets with faster growth time-scales. Lastly, we find that the low viscosities necessary for vortex formation likely prevent planets from growing quickly enough to trigger the instability in self-consistent models.

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.