The Heliocentric Evolution of Key Species in the Distantly-Active Comet C/1995 O1 (Hale-Bopp)

Comet Hale-Bopp (C/1995 01) presents the first opportunity to study physical and chemical changes in the coma of a bright, active comet at large heliocentric distances with modern millimeter-wavelength spectrometers. We obtained mm-wave spectra of CH₃OH, CO, and H2CO in Hale-Bopp on 29 dates when it was beyond 4 AU. We report the first detection of CH₃OH in Hale-Bopp at 5.0 AU. Additionally, we found that CO production went through three distinct phases: (i) an initial, rapid increase, (ii) quasi-saturation, and then (iii) a renewed, but less rapid increase. The abrupt renewal of the CO production rate increase took place near both the onset of H₂O sublimation and the first detections of CH₃OH and HCN, suggesting a causal relationship. Line shape and velocity offset information in our spectra give important clues to the production mechanisms of the three species. Linewidths were relatively narrow (0.3 km s^-1) for CO and much wider (1-2 km s^-1) for CH₃OH and H₂CO. Line profiles of CO were substantially blueshifted with respect to the comet's frame of reference by an average of 0.3 km s^-1, while CH₃OH and H₂CO emission appeared essentially at rest with respect to the comet's frame. Based on these findings, we infer that the CO we observed was produced primarily at the nucleus in jets, rather than from an extended source, and that the majority of both the CH₃OH and H₂CO appear instead to be released as daughter products from the dissociation of heavier molecules.