SPECTROSCOPIC OBSERVATIONS OF COMET C-1996-B2 (HYAKUTAKE) WITH THE CALTECH SUBMILLIMETER OBSERVATORY

The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected an d rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity i ts chemical composition, We present observations of over 30 submillime ter transitions of HCN, (HCN)-C-13, HNC, HNCO, CO, CH3OH, and H2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the C altech Submillimeter Observatory, Detections of the (HCN)-C-13 (4-3) a nd HNCO (16(0,16)-15(0,15)) transitions represent the first observatio ns of these species in a comet. In addition, several other transitions , including HCN (8-7), CO (4-3), and CO (6-5) are detected for the fir st time in a comet as is the hyperfine structure of the HCN (4-3) line . The observed intensities of the HCN (4-3) hyperfine components indic ate a line center optical depth of 0.9 +/- 0.2 on March 22.5 UT. The H CN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance o f 1 AU is similar to that measured in the Orion extended ridge-a warm, quiescent molecular cloud, The HCN/(HCN)-C-13 abundance ratio implied by our observations is 34 +/- 12, similar to that measured in giant m olecular clouds in the galactic disk but significantly lower than the Solar System C-12/C-13 ratio. The low HCN/(HCN)-C-13 abundance ratio m ay be in part due to contamination by an SO2 line blended with the (HC N)-C-13 (4-3) line, In addition, chemical models suggest that the HCN/ (HCN)-C-13 ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/(HCN)-C-13 ratio obser ved in a comet is not inconsistent with the solar system C-12/C-13 iso topic ratio, The abundance of HNCO relative to water derived from our observations is (7 +/- 3) x 10(-4), The HCN/HNCO abundance ratio is si milar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO prod uced by HNCO is a negligible component of cometary atmospheres. Produc tion rates of HCN, CO, H2CO, and CH3OH are presented, Inferred molecul ar abundances relative to water are typical of those measured in comet s at 1 AU from the Sun. The exception is CO, for which we derive a lar ge relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus, The time evolution of the H2CO emission suggests production of this species fr om dust grains. (C) 1997 Academic Press.