Millimeter-wave molecular mapping of comets Hyakutake and Hale-Bopp
The inner solar system passes of the bright comets C/1995 O1 Hale-Bopp and C/1996 B2 Hyakutake, coupled with modern, sensitive millimeter-wave instrumentation provided an opportunity to make unprecedented maps of molecular emission from cometary comae. We present the results of the 3-mm observing campaign to study these comets at the Five College Radio Astronomy Observatory (FCRAO). Observations of both comets in the HCN J=1 − 0 rotational transition provide a good record of the emission over a long period of time. The HCN production rates of comet Hyakutake are consistent with the observed pre-perigee visual brightening, concurrent with obvious spatial asymmetries in the maps. Monte Carlo simulations of the quiescent comet achieve best results with collision-dominated excitation over the observed coma, and a variation in outflow velocity with cometocentric distance. Simulations during the time of the outburst reveal that local enhancements in the emission, up to a factor of 5 over the isotropic outgassing, can account for the observed features. Searches for 3-mm. molecular lines of CN, HC 3N, and CH3OH were conducted, and upper limits are presented. Similar observations and modeling of comet Hale-Bopp confirm the need for modeling a gradient in the coma outflow velocity, and shed further light on how spectral line maps are affected by the presence of spatially asymmetric emission.
Comet Hale-Bopp was also mapped in the 3-mm emission of the ion HCO +. HCO+ is detectable over an extended region at the comet, with the peak emission commonly located 50,000–100,000 km in the antisolar direction. Maps made throughout the apparition show significant time-variability in the structure of the HCO+ coma; however, the bulk properties of the emission remain constant. The HCO+ brightness is usually depressed at the nucleus position, and on some occasions, the emission is spread into a ring or horseshoe. Individual spectra within the maps display broad lines redshifted from the nominal velocity of the nucleus, with the redshift typically increasing in the antisolar direction. The spectra and maps may be generally explained by models in which the ions are accelerated tailward at a rate ∼10 CM s−2, provided that HCO+ is destroyed near the nucleus. This destruction is presumably achieved via ion-molecule chemical reactions.