Millimeter/submillimeter Observations of Chemical and Physical Complexity in Protoplanetary Disks
Loomis, Ryan Alexander
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AbstractDust and gas rich protoplanetary disks are the formation sites of planets, and are virtually ubiquitous around young (<1 Myr) stars. Understanding these environments is crucial to determining future exoplanet compositions, as well as uncovering the origin of Earth's organic reservoir. In this thesis, I present interferometric radio observations from the Atacama Large Millimeter Array (ALMA) designed to study the signatures of chemical and physical complexity in protoplanetary disks, as well as new analysis techniques that significantly increase the utility of these observations.
The high sensitivity of ALMA has enabled the study of complex organics in protoplanetary disks. H2CO represents an important stepping stone on the path to more complex organics such as CH3OH, but its utility as a tracer of grain-surface hydrogenation is limited by the presence of competing gas-phase formation pathways. I use archival ALMA observations of resolved H2CO emission around the T Tauri star DM Tau to assess the relative importance of gas-phase vs. grain-surface formation. I show that the emission can be separated into two spatially resolved components, with gas-phase reactions producing a central peak and grain-surface chemistry responsible for emission exterior to the CO snowline. Similarly, I analyze observations of the complex organic CH3CN in a protoplanetary disk around the T Tauri star TW Hya, a Solar Nebula analog, and place initial constraints on its distribution and formation chemistry. Through comparison of the observations with predictions from a disk chemistry model, I find that grain-surface reactions likely dominate CH3CN formation. Comparing this model with Solar System cometary measurements, I show that either vigorous vertical mixing or some degree of inheritance from interstellar ices likely occurred in the Solar Nebula.
A major challenge in analyzing observations of trace species in disks is that their emission is often very weak. I present a new method for detecting weak spectral lines in interferometric data through the use of matched filters. I demonstrate how a-priori knowledge of source structure can be used to construct an approximated matched filter, and show that applying this filter in the native uv plane yields optimal detection efficiency, with possible SNR boosts of over <500%. Utilizing this technique, I analyze a spectral survey of two protoplanetary disks around the Herbig Ae star MWC 480 and the T-Tauri star LkCa 15, covering ~36 GHz nearly continuous bandwidth in ALMA Band 7. I detect five new species in disks for the first time: 13CS, C34S, H2CS, DNC, and C2D, and I show how differences in radiation environment affect the molecular inventories around the two stars.
Finally, ALMA has recently begun to reveal a wealth of sub-structure in the mm dust continua of disks, which may be related to the concurrent process of planet formation. I present high resolution (~0.2'') ALMA observations of the 1.3mm dust continuum emission from the disk around T Tauri star AA Tau that identify it for the first time as multi-ringed system viewed at a moderate inclination of 59 +/- 2 degrees, challenging previous assumptions of an edge-on disk. I identify non-axisymmetric features in the dust disk and used the projected velocity field of HCO+ emission to suggest the presence of a misalignment between the inner and outer disks. To better image such features, I present a new adaptive weighting scheme for interferometric imaging, which fully generalizes traditional Briggs 'robust' weighting. I show that image fidelity is improved for all interferometric imaging cases, including protoplanetary disks.
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