Thesis (Ph.D.) - University of St Andrews, November 2008.

abstract: The composition of planets and their volatile contents are intimately connected to the structure and evolution of their parent protoplanetary disks. The transport of momentum and volatiles is often parameterized by a turbulent viscosity parameter $\alpha$, which is usually assumed to be spatially and temporally uniform across the disk. I show that variable $\alpha$(r,z) (where $r$ is radius, and $z$ is height from the midplane) attributable to angular momentum transport due to MRI can yield disks with significantly different structure, as mass piles up in the 1-10 AU region resulting in steep slopes of p $>$ 2 here (where p is the power law exponent in $\Sigma \propto r^{-p}$). I also show that the transition radius (where bulk mass flow switches from inward to outward) can move as close in as 3 AU; this effect (especially prominent in externally photoevaporated disks) may significantly influence the radial water content available during planet formation. I then investigate the transport of water in disks with different variable α profiles. While radial temperature profile sets the location of the water snowline (i.e., inside of which water is present as vapor; outside of which, as ice on solids), it is the rates of diffusion and drift of small icy solids and diffusion of vapor across the snow line that determine the radial water distribution. All of these processes are highly sensitive to local $\alpha$. I calculate the effect of radially varying α on water transport, by tracking the abundance of vapor in the inner disk, and fraction of ice in particles and larger asteroids beyond the snow line. I find one α profile attributable to winds and hydrodynamical instabilities, and motivated by meteoritic constraints, to show considerable agreement with inferred water contents observed in solar system asteroids. Finally, I calculate the timing of gap formation due to the formation of a planet in disks around different stars. Here, I assume that pebble accretion is the dominant mechanism for planetary growth and that the core of the first protoplanet forms at the water snow line. I discuss the dependence of gap timing to various stellar and disk properties. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2018

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (p. 103-106).

Class I protostars are thought to represent an early stage in the lifetime of protoplanetary disks, when they are still embedded in their natal envelope. Here we measure the disk masses of 10 Class I protostars in the Taurus Molecular Cloud to constrain the initial mass budget for forming planets in disks. We use radiative transfer modeling to produce synthetic protostar observations and fit the models to a multi-wavelength data set using a Markov Chain Monte Carlo fitting procedure. We fit these models simultaneously to our new Combined Array for Research in Millimeter-wave Astronomy 1.3 mm observations that are sensitive to the wide range of spatial scales that are expected from protostellar disks and envelopes so as to be able to distinguish each component, as well as broadband spectral energy distributions compiled from the literature. We find a median disk mass of 0.018 M-circle dot on average, more massive than the Taurus Class II disks, which have median disk mass of similar to 0.0025 M-circle dot. This decrease in disk mass can be explained if dust grains have grown by a factor of 75 in grain size, indicating that by the Class II stage, at a few Myr, a significant amount of dust grain processing has occurred. However, there is evidence that significant dust processing has occurred even during the Class I stage, so it is likely that the initial mass budget is higher than the value quoted here.

This thesis has investigated the chemical evolution of the inner regions (r ≤ 10 AU) of a modelled protoplanetary disk surrounding a low-mass T Tauri star; a phase our own solar system underwent some 4.5–4.6 billion years ago. A 1+1- dimensional physical model of a radially-accreting protoplanetary disk was combined with a chemical model consisting of gas-phase reactions extracted from the RATE95 UMIST database for Astrochemistry, and to this, a network of gas-grain interaction and deuterated reactions were added. Influenced by the knowledge that radionuclides may have been abnormally abundant in our early solar system compared with the interstellar medium, and that the energy expelled from their decay is sufficient to ionize molecules, a 1-dimensional simulation along the disk midplane was performed, comparing the chemistry with and without radionuclides, as a function of the radionuclide ionization rate. The molecules C4H2, HC3N, C3H, HCN, CH4, C2H2 and N2 were found to be particularly sensitive. Of these, HCN and C2H2 have already been detected in protoplanetary disks. Motivated by observations which suggest that T Tauri systems vary from faster to slower accretion rates, the chemical distributions of two disks with stellar accretion rates of 10−7 M⊙ yr−1 and 10−8 M⊙ yr−1 were compared. Allowing the mass accretion rate (and thus physical conditions) to vary in time, starting from 10−7 M⊙ yr−1, and evolving to a mass accretion rate of 10−8 M⊙ yr−1, the molecules CN, HCN, H2CO and NH3 were found to be particularly sensitive when compared to a standalone 10−8 M⊙ yr−1 simulation. With the use of a 1-D CASA LTE algorithm for ALMA, the sensitivities of HCN and H2CO were transcribed into integrated intensity differences, as a function of emissivity and optical depth. The largest differences were associated with the largest feasible transitions of HCN and H2CO; J = 8-7 and JK = 10010-909 respectively, but could not be converted into potentially observable integrated fluxes due to the restrictions of this model. Both molecules were found to trace different regions of the disk. Using the stellar accretion rate-to-age linear relation evaluated from observations, the calculation times for the 10−7 M⊙ yr−1 and 10−8 M⊙ yr−1 were re-evaluated, of which CN and NH3 emerged as the most sensitive molecules. Thus, CN and NH3 may be a possible tracer of calculation time, and disk age.

In this thesis I have studied how discs around young stars evolve and disperse. In particular, I build models which combine viscous evolution with photoevaporation, as previous work suggests that photoevaporation can reproduce the observed disc evolution and dispersal time-scales. The main question this thesis attempts to address is: Can photoevaporation provide a dominant dispersal mechanism for the observed population of young stars? Photoevaporation arises from the heating that high energy (UV and X-ray) photons provide to the surface layers of a disc. Before I started this work, only photoevaporation from a pure EUV radiation field was described within a hydrodynamic framework. Therefore, I start by building a hydrodynamic solution to the pure X-ray photoevaporation problem, and then extend this solution to the entire high energy spectrum. This hydrodynamic model leads me to conclude that it is the X-ray radiation field that sets the mass-loss rates. These mass-loss rates scale linearly with X-ray luminosity, are independent of the underlying disc structure and explicitly independent of stellar mass. I build a radiation-hydrodynamic algorithm, based on previous work, to describe the process of X-ray heating in discs. I then use this algorithm to span the full range of observed parameter space, to fully solve the X-ray photoevaporation problem. I further extend the algorithm to roughly approximate the heating an FUV radiation field would have on the photoevaporative flow, as well as separately testing the effect an EUV radiation field will have. These numerical tests are in agreement with the hydrodynamic model derived. Specifically, it is the X-rays that are driving the photoevaporative flow from the inner disc. Armed with an accurate description of the photoevaporative mass-loss rates from young stars, I consider the evolution of a population of disc-bearing, young (

In recent years, thousands of exoplanets have been discovered around a variety of stellar hosts. The disks of gas and dust surrounding young stars are the location and source of material for planet formation. The properties of these protoplanetary disks therefore directly affect the planetary systems that may form. However, the details of the planet formation process are still unclear. In this dissertation, I constrain planet formation mechanisms by measuring the properties of protoplanetary disks, focusing on mass, dust grain growth, and dust settling. I use physically-motivated models and an Artificial Neural Network along with a Markov Chain Monte Carlo (MCMC) fitting procedure to obtain these and other disk properties. This dissertation compiles the largest sample to date of consistently-modeled protoplanetary disks, probing how disk properties vary with host mass and age. The occurrence of planetary companions increases as stellar mass decreases. Thus, brown dwarfs (BDs), with smaller masses than pre-main-sequence stars, may commonly host planets. Studying properties of BD disks and comparing them to pre- main-sequence star disks is therefore important for constraining their planet-forming potential. I present spectral energy distribution (SED) models for BD and pre-main- sequence star disks in four star-forming regions. The SEDs consist of archival photometry data spanning optical to millimeter wavelengths. I model the BD disk SEDs using physically-motivated radiative transfer code; pre-main-sequence star SEDs are modeled using a newly-developed MCMC fitting procedure that allows for a more complete analysis of the disk properties. I compare disk masses and dust settling in these two disk categories to gauge how host mass affects these properties. Typical disk lifetimes are a few tens of millions of years; planet formation likely occurs within the first few million years or less. Comparing how disk properties vary between star-forming regions of different ages can help pinpoint the timeline for planet formation. I present SED models for BDs in four star-forming regions and pre-main-sequence stars in eleven star-forming regions. I obtain the disk masses, dust grain sizes, and amount of dust settling in the disks and discuss the differences and similarities of these properties across regions of varying age.

We present Karl G. Jansky Very Large Array 1.3, 3.6, and 6 cm continuum maps of compact radio sources in the Orion Nebular Cluster (ONC). We mosaicked 34 arcmin(2) at 1.3 cm, 70 arcmin(2) at 3.6 cm and 109 arcmin(2) at 6 cm, containing 778 near-infrared detected young stellar objects and 190 Hubble Space Telescope-identified proplyds (with significant overlap between those characterizations). We detected radio emission from 175 compact radio sources in the ONC, including 26 sources that were detected for the first time at these wavelengths. For each detected source, we fitted a simple free-free and dust emission model to characterize the radio emission. We extrapolate the free-free emission spectrum model for each source to ALMA bands to illustrate how these measurements could be used to correctly measure protoplanetary disk dust masses from submillimeter flux measurements. Finally, we compare the fluxes measured in this survey with previously measured fluxes for our targets, as well as four separate epochs of 1.3 cm data, to search for and quantify the variability of our sources.

We study the formation of planetesimals in protoplanetary disks from the gravitational collapse of solid over-densities generated via the streaming instability. To carry out these studies, we implement and test a particle-mesh self-gravity module for the ATHENA code that enables the simulation of aerodynamically coupled systems of gas and collisionless self-gravitating solid particles. Upon employment of our algorithm to planetesimal formation simulations, we find that (when a direct comparison is possible) the ATHENA simulations yield predicted planetesimal properties that agree well with those found in prior work using different numerical techniques. In particular, the gravitational collapse of streaming-initiated clumps leads to an initial planetesimal mass function that is well-represented by a power law, dN / dM(p) proportional to M-p(-p), with p similar or equal to 1.6 +/- 0.1, which equates to a differential size distribution of dN / dR(p) proportional to R-p(-q), with q similar or equal to 2.8 +/- 0.1. We find no significant trends with resolution from a convergence study of up to 512(3) grid zones and N-par approximate to 1.5 x 10(8) particles. Likewise, the power-law slope appears indifferent to changes in the relative strength of self-gravity and tidal shear, and to the time when (for reasons of numerical economy) self-gravity is turned on, though the strength of these claims is limited by small number statistics. For a typically assumed radial distribution of minimum mass solar nebula solids (assumed here to have dimensionless stopping time tau = 0.3), our results support the hypothesis that bodies on the scale of large asteroids or Kuiper Belt Objects could have formed as the high-mass tail of a primordial planetesimal population.

Context. Understanding the diversity of planets requires studying the morphology and physical conditions in the protoplanetary disks in which they form. Aims. We aim to study the structure of the similar to 10 Myr old protoplanetary disk HD 100453, to detect features that can trace disk evolution and to understand the mechanisms that drive these features. Methods. We observed HD100453 in polarized scattered light with VLT/SPHERE at optical (0.6 mu m, 0.8 mu m) and near-infrared (1.2 mu m) wavelengths, reaching an angular resolution of similar to 0.02 '', and an inner working angle of similar to 0.09 ''. Results. We spatially resolve the disk around HD 100453, and detect polarized scattered light up to similar to 0.42 '' (similar to 48 au). We detect a cavity, a rim with azimuthal brightness variations at an inclination of similar to 38 degrees with respect to our line of sight, two shadows and two symmetric spiral arms. The spiral arms originate near the location of the shadows, close to the semi major axis. We detect a faint feature in the SW that can be interpreted as the scattering surface of the bottom side of the disk, if the disk is tidally truncated by the M-dwarf companion currently seen at a projected distance of similar to 119 au. We construct a radiative transfer model that accounts for the main characteristics of the features with an inner and outer disk misaligned by similar to 72 degrees. The azimuthal brightness variations along the rim are well reproduced with the scattering phase function of the model. While spirals can be triggered by the tidal interaction with the companion, the close proximity of the spirals to the shadows suggests that the shadows could also play a role. The change in stellar illumination along the rim induces an azimuthal variation of the scale height that can contribute to the brightness variations. Conclusions. Dark regions in polarized images of transition disks are now detected in a handful of disks and often interpreted as shadows due to a misaligned inner disk. However, the origin of such a misalignment in HD100453, and of the spirals, is still unclear, and might be due to a yet-undetected massive companion inside the cavity, and on an inclined orbit. Observations over a few years will allow us to measure the spiral pattern speed, and determine if the shadows are fixed or moving, which may constrain their origin.