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Chunhua Qi (Caltech & Harvard-Smithsonian CfA)
Improvements in observational methods have now made it possible to directly observe planet-forming environments around nearby young stars and to better characterize the most primitive relics of planetary growth in our own solar system. This thesis describes one such method, aperture synthesis imaging using the Owens Valley Radio Observatory (OVRO) Millimeter Array, and its application to the chemical composition of circumstellar accretion disks and comets. The OVRO observations of disks concentrate on 3 mm transitions of HCN/HCO+ and 13CO/CN toward the star/disk systems LkCa 15, GM Aur, MWC 480, and HD 163296. Even at a resolution of 2'' (or a linear scale of ~300 AU at the distance of Taurus and Ophiuchus), the OVRO observations show that the chemistry in circumstellar disks is sensitive to both the central stellar lumninosity and the degree of dust settling toward the disk midplane. The emission from LkCa 15 is particularly intense, with many molecules being detected, including HCN/HCO+ and their 13C-isotopomers, DCN, CN, CH3OH, CS, SO, 13CO,and C18O. The overall abundance patterns are consistent with recent models of photon-dominated chemistry in the near surface regions of flaring circumstellar disks that also provide a natural explanation for the mid- and far-infrared properties of the disk spectral energy distribution.
Direct ties between accretion disks and the formation of planetary systems can be tested by examining primitive solar system bodies such as comets. Comet Hale-Bopp was observed at OVRO between 1997 March 29 and April 2 in a variety of spectroscopic settings between 3.4 and 1.2 mm. The resulting aperture synthesis continuum and molecular line images reveal in great detail the inner coma. The OVRO Millimeter Array was able to image, for the first time, molecular analogs of the dust jets commonly observed in the optical and infrared. This is particularly significant for investigating the true composition of comets. While the substantial D/H fractionations suggests an evolutionary history in which cometary materials remain at very low temperatures throughout their assemblage, the complex chemistry revealed in the OVRO images of the cold, outer regions of disks around young stars means that it is difficult to characterize cometary volatiles as being primarily ``interstellar'' or ``nebular'' in origin.
The OVRO Millimeter Array is operated by the California Institute of Technology under funding from the National Science Foundation (AST99-81546). Additional support from the NASA Origins of Solar Systems Program is gratefully acknowledged.
The author(s) of this abstract have provided an email address for comments about the abstract: cqi@cfa.harvard.edu