CU Dissertations

The Distribution of Volatiles in Protoplanetary Disks
Degree Awarded: Ph.D. Physics. The Catholic University of America, 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.
Monostatic all-fiber LADAR systems
Degree Awarded: Ph.D. Electrical Engineering and Computer Science. The Catholic University of America, Many applications require a LADAR system smaller in size and more power efficient than those using a polygon, galvo, or Risley-prism beam scanner. A fiber-coupled, monostatic LADAR system which transmits and receives through the same aperture has many advantages. These advantages include low cost, easy optical alignment, small size, and low weight. Optical alignment of the system is greatly simplified since there is no parallax between transmitted and received beam paths. The direction of the received light is inherently aligned with that of the transmitted beam. Multiple alignment steps of bulk optical components are one of the major reasons many laser systems are expensive. Using an all-fiber approach the optical alignment is simplified by automated splicing of the fibers. A 1-D LADAR consisting of a stationary monostatic rangefinder with real-time pulse processing is first demonstrated. Then a 3-D scanning LADAR is demonstrated. A vibrating fiber cantilever tip that is used to scan the laser beam is mounted on a resonant piezo-electric lead zirconium titanate (PZT) stripe actuator that requires very little power. A position sensing detector (PSD) is needed to sense the fiber position since the motion of the scanned fiber may not be fully predictable. The PSD operates in a two-photon absorption mode to accurately measure the fiber tip position for each laser pulse, with very low insertion loss.
Study of Photospheric Magnetic Field and Coronal Emission from Active Regions
Degree Awarded: Ph.D. Physics. The Catholic University of America, Solar explosive phenomena (flares and Coronal Mass Ejections, CMEs) are examples of how the most dynamical processes within the heliosphere are interconnected and powered by the Sun. Solar flares originate in active regions (AR) -- areas of strong magnetic field on the solar surface. The electromagnetic (EM) energy released during flares, along with the often-seen CMEs, propagate through the heliosphere. In the Earth's vicinity, EM radiation and charged particles have the potential to produce unfavorable conditions for humans and technology in space. From many points of view (scientific, operational, economical) it is thus important to understand and try to predict when solar flares and associated eruptive phenomena will occur. This dissertation explores how to best leverage the available observational data to provide predictive information about the future flaring activity. This dissertation consists of two main components: 1) investigation of the photosphere-corona coupling by analyzing photospheric magnetic field and coronal data in search for signals or behaviors that precede eruptions; and 2) the combination of existing flare prediction methods in order to develop a novel ensemble prediction. For the first part, the data employed correspond to line-of-sight (LOS) magnetograms from the Helioseismic and Magnetic Imager (HMI) and EUV intensity maps from the Atmospheric Imaging Assembly (AIA), both instruments onboard NASA's Solar Dynamics Observatory (SDO) satellite. Photospheric magnetic field and coronal EUV emissions were characterized by measuring the power-law decay of their spatio-temporal spectra and the data statistical associations (auto- and cross-correlations). These measures, calculated with high spatio-temporal resolution, appeared to characterize the AR evolution, provide information about the state of the photospheric plasma, reveal insights into the photospheric conditions for flares, and expose the potential of combining coronal and photospheric data for flare prediction. On the other hand, the construction of an ensemble forecast for major flares (M and X classes) was carried out by assigning a statistical weight to each ensemble member -- calculated from their track history and then linearly combining the forecasts. The performed linear combination resulted in the improvement of both probabilistic and categorical forecast by carefully choosing the performance metric to optimize. The performance of the ensemble forecasting method could be further improved by expanding the training sample of forecasts and observations. In conclusion, the combination of high-quality observational data with advanced statistical analysis techniques holds the key for understanding the photosphere-corona coupling and release of magnetic energy in solar flares. It is possible to enhance the quality of the existing flare forecasts by using smart ways to combine information from a set of different methods.