Aperture synthesis studies of the chemical composition of protoplanetary disks and comets

Abstract

Improvements in observational methods have now made it possible to directly observe planet-forming environments around 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 observations presented in this thesis concentrate on [lambda]~3 mm transitions of HCN/HCO+ and {13}CO/CN in LkCa 15, GM Aur, MWC 480, and HD 163296. These disks were chosen based on their large spatial extent, Keplerian kinematic patterns, and strong CO emission. Even at a resolution of (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 luminosity and the degree of dust settling toward the disk midplane. Abundance ratios such as CN/HCN and HCO+/CO serve as unique probes of pivotal processes such as photoevaporation or cosmic ray induced ionization. The observed lower limit to the fractional ionization of 10{-10} is sufficient to support magnetorotational instabilities that are likely to dominate the transport mechanisms in the outer reaches of protoplanetary disks. CN/HCN gradients in the T Tauri and Herbig Ae star disks appear to be correlated with the local UV radiation field and with the degree of dust settling as judged by recent fits to their spectral energy distributions, illustrating the important role of photochemistry at large disk radii. The disk emission toward LkCa 15 is particularly intense, with many molecules being detected, including HCN/HCO+ and their {13}C-isotopomers, DCN, CN, HC_3N, CH_3OH, CS, {13}CO, and C{18}O. 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 from 1997 March 29 to April 2 in a variety of spectroscopic settings between 3.4 and 1.2 mm. The resulting aperture synthesis millimeter-wave continuum and molecular line images reveal in great detail the inner coma. The millimeter-wave continuum brightness of Hale-Bopp is dominated by emission from dust grains in the coma. By subtracting a spherically symmetric coma model from the continuum visibilities, the millimeter-wave flux from the nucleus has been isolated, and leads to an estimated radius of 19-22.5 km. The large size of comet Hale-Bopp accounts for its extraordinary outgassing rates, which permitted the aperture synthesis observations of over 18 millimeter transitions of HCN, DCN, HDO, HC_3N, HNC, HNCO, CS, H_2S}, SO, OCS, CO, CH_3OH and CH_3OCH_3. The OVRO Millimeter Array was able to image, for the first time, molecular analogs of the dust jets commonly observed at optical and infrared wavelengths. This is particularly significant for investigating the true composition of comets, since jets are known to lift off large, icy grains from which direct sublimation can occur as they are exposed to the Sun. The production rates derived from the aperture synthesis images are similar to those found by other researchers, and reveal a marked similarity between the composition of Hale-Bopp and that derived for dense molecular clouds, in particular the hot cores observed near massive young stars. In addition, quite substantial D/H fractionations, comparable to the OVRO DCN/HCN measurement in LkCa 15, are found in the jets. While this clearly suggests an evolutionary history in which cometary materials remain at very low temperatures throughout their assemblage and for the bulk of their lives, the complex, kinetically controlled 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.