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[41.15] The Fate of Comet C/1999 S4 (LINEAR) at Radio Wavelengths

N. Biver (obs. de Paris & IfA, Hawaii), D. Bockelée-Morvan, P. Colom, J. Crovisier, E. Gérard, F. Henry (obs. de Paris), R. Moreno (IRAM), D.C. Lis (Caltech), H.E. Matthews (JAC, Hawaii), J. Bauer (IfA, Hawaii), D. Despois (obs. de Bordeaux)

Comet C/1999 S4 (LINEAR) was observed at radio wavelengths using the Caltech Submillimeter Observatory (CSO), James Clerk Maxwell Telescope (JCMT) (Mauna-Kea) and Institut de RadioAstronomie Millimétrique (IRAM) 30-m millimeter and submillimeter radio telescopes and Nançay decimeter radio telescope between January 3 and August 2, 2000. HCN, CS, H₂CO and OH lines were clearly detected in June-July 2000. Marginal detections or significant upper limits were obtained for H₂S, HNC, CH₃OH and CO. This chemical investigation suggests that this comet was particularly depleted in methanol (CH₃OH/H₂O < 1%).

The most striking information from nearly daily observations of the comet between July 6 and 30 are the dramatic variations of the outgassing rates during this period. An outburst was observed on July 23.7 followed by a steep decrease of activity. This evolution is presumably related to the disruption of the nucleus observed at visible wavelengths (IAUC 7468 and following). Nançay observations show a water production rate of \approx 3\times10²⁸ molec. s^-1 on July 6--9. Afterwards, only a marginal detection of the OH lines could be achieved on July 12--20.

The IRAM monitoring showed an increase of the HCN outgassing rate by a factor \approx 6 between July 18.7 and July 23.7, followed by a steep decrease by a factor \approx 60 in only 3 days. The HCN lines could not be detected at any telescope (IRAM, CSO or JCMT) after July 27, suggesting that most of the remaining nucleus volatiles were dispersed at the time of the outburst.

Further details on the chemical composition and outgassing behavior of the comet will be presented.