Spectroscopic Observations of Comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory

TitleSpectroscopic Observations of Comet C/1996 B2 (Hyakutake) with the Caltech Submillimeter Observatory
Publication TypeJournal Article
Year of Publication1997
AuthorsLis, D. C., Keene J., Young K., Phillips T. G., Bockelée-Morvan D., Crovisier J., Schilke P., Goldsmith P. F., and Bergin E. A.
Date PublishedDec

The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H\^ 13CN, HNC, HNCO, CO, CH\_3OH, and H\_2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H\^ 13CN (4-3) and HNCO (16\_0,16-15\_0,15) transitions represent the first observations 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 first 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 indicate a line center optical depth of 0.9 +/- 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge- a warm, quiescent molecular cloud. The HCN/H\^ 13CN abundance ratio implied by our observations is 34 +/- 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System \^ 12C/\^ 13C ratio. The low HCN/H\^ 13CN abundance ratio may be in part due to contamination by an SO\_2 line blended with the H\^ 13CN (4-3) line. In addition, chemical models suggest that the HCN/H\^ 13CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H\^ 13CN ratio observed in a comet is not inconsistent with the solar system \^ 12C/\^ 13C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 +/- 3) x 10\^ -4. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H\_2CO, and CH\_3OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large 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 H\_2CO emission suggests production of this species from dust grains.