The Distribution of Volatiles in Protoplanetary Disks
This dissertation addresses several distinct goals, all relating to the distribution of molecular gas in planet-forming regions of protoplanetary disks surrounding young, Sun-like stars. First, water surface snow line radii are measured for four classical protoplanetary disks. These measurements have been theoretically predicted using hydrodynamic and radiative transfer models but with contrasting results (Kennedy and Kenyon, 2008; Martin and Livio, 2013), and until now have not yet been measured using observations. Next a detailed analysis of the inner disk chemistry of one classical disk from the sample is presented and the long-standing question is addressed: to what degree is interstellar chemistry inherited by planet-forming material, or are protoplanetary disks themselves intrinsic "chemical factories" (Pontoppidan et al., 2014; van Dishoeck et al., 2014).Finally, rare CO isotopologue emission in the inner disk region of one transitional disk is presented. Transitional disks have inner regions heavily depleted in small dust grains, with known gaps in their dust structures. Estimates of the inner and outer disk gas-to- dust mass ratios may support evidence for dust clearing due to grain growth, and/or developing planetesimals.Two-dimensional radiative transfer modeling is used to retrieve dust density and temperature structures and render infrared line spectra. These models constitute modeling grids used to fit a parameterized gas distribution to the data. Best-fit model parameters determine the radial location where water vapor is observed to decrease by orders of magnitude, thus allowing the retrieval of surface snow line locations in the classical disk sample. This modeling framework is also applied to retrieve the molecular concentrations of H2O, CO, HCN, C2H2, and CO2 for one classical disk in the same sample, so that its inner disk chemistry can be compared to that of cold molecular clouds and solar system comets (Pontoppidan and Blevins, 2014). Lastly, the dust modeling framework is applied to observations of the transitional disk, SR 21, to estimate gas-to-dust mass ratios in the inner planet-forming region and beyond.Results are obtained addressing each research goal. First, water surface snow lines are measured at ~ 3 - 11 AU for the classical disk sample. Assuming a canonical gas-to-dust ratio of 100, as well as coupled gas and dust temperatures, the best-fit inner water abundances become implausibly high (0.01-1.0 per H2). Conversely, models in which the gas and dust temperatures are decoupled leads to canonical inner disk water abundances of ~ 10-4 per H2, while retaining gas-to-dust ratios of 100. That is, the evidence for gas-dust decoupling in disk surfaces is stronger than for enhanced gas-to-dust ratios.Next concentrations of dominant molecular carriers of carbon, oxygen, and nitrogen in the terrestrial region around 1 AU for classical disk RNO 90 were retrieved and compared to the chemical inventory of dense clouds and protostellar envelopes. The results argue that inner disk chemistry is, as expected, fundamentally different from prestellar chemistry. The clearest discriminant is the concentration of CO2, which is extremely low in disks, but one of the most abundant constituents of dense clouds and protostellar envelopes.Lastly, the gas-to-dust mass ratio in the inner planet-forming region (less than or equal to 10 AU) of the SR 21 transitional disk is estimated. The best-fit dust continuum model results require dust depletion along with an increased gas-to-dust ratio in the inner disk to adequately model the observations in the near- to mid-infrared wavelength regions. The outer disk region can be modeled without dust depletion using a canonical gas-to-dust ratio of 100. Finally ideas for future work, addressing each research goal, is presented.
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