The properties of planet-forming midplanes of protostellar disks remain largely unprobed by observations due to the high optical depth of common molecular lines and continuum. However, rotational emission lines from rare isotopologues may have optical depth near unity in the vertical direction, so that the lines are strong enough to be detected, yet remain transparent enough to trace the disk midplane. In this thesis, we present a chemical model of an MRI-active protoplanetary disk including different C, O isotopes and detailed photochemical reactions. The CO condensation front is found to be at 1.5 AU on the disk midplane around a solar like star, and its location remains almost unchanged during 3Myr of evolution. The optical depth of low-order rotational lines of C¹⁷O are around unity, which suggests it may be possible to see into the disk midplane using C¹⁷O. Such ALMA observations would provide estimates of the disk midplane temperature if the CO ice lines were spatially or spectrally resolved. With our computed C¹⁷O/H₂ abundance ratio, one would also be able to measure the disk masses by measuring the intensity of gas emission. / text

Most stars are born in multiple systems, so the presence of a stellar companion may commonly influence planet formation. Theory indicates that companions may inhibit planet formation in two ways. First, dynamical interactions can tidally truncate circumstellar disks. Truncation reduces disk lifetimes and masses, leaving less time and material for planet formation. Second, these interactions might reduce grain-coagulation efficiency, slowing planet formation in its earliest stages. / Astronomy

Thesis (Ph.D.) - University of St Andrews, March 2007.

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.

<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)

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.

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.

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.

Context. The presence or absence of ice in protoplanetary disks is of great importance to the formation of planets. By enhancing solid surface density and increasing sticking efficiency, ice catalyzes the rapid formation of planetesimals and decreases the timescale of giant planet core accretion. Aims. In this paper, we analyze the composition of the outer disk around the Herbig star HD 142527. We focus on the composition of water ice, but also analyze the abundances of previously proposed minerals. Methods. We present new Herschel far-infrared spectra and a re-reduction of archival data from the Infrared Space Observatory (ISO). We modeled the disk using full 3D radiative transfer to obtain the disk structure. Also, we used an optically thin analysis of the outer disk spectrum to obtain firm constraints on the composition of the dust component. Results. The water ice in the disk around HD 142527 contains a large reservoir of crystalline water ice. We determine the local abundance of water ice in the outer disk (i.e., beyond 130AU). The re-reduced ISO spectrum differs significantly from that previously published, but matches the new Herschel spectrum at their common wavelength range. In particular, we do not detect any significant contribution from carbonates or hydrous silicates, in contrast to earlier claims. Conclusions. The amount of water ice detected in the outer disk requires similar to 80% of oxygen atoms. This is comparable to the water ice abundance in the outer solar system, comets, and dense interstellar clouds. The water ice is highly crystalline while the temperatures where we detect it are too low to crystallize the water on relevant timescales. We discuss the implications of this finding.