Millimetre Observations of Comets P/Brorsen-Metcalf (1989o) and Austin (1989c1) With the IRAM 30-m Radio Telescope

Millimetre observations with the IRAM 30-m telescope were conducted in comet PBrorsen-Metcalf (1989o) on September 1989 and Austin (1989cl) on April and May 1990. The HCN J(1-0) and J(3-2) lines were detected in both comets. The HCN production rate relative to water in P/Brorsen-Metcalf is comparable to that previously measured in comet P/Halley, white that inferred in comet Austin might be smaller by a factor of two. The H2CO 312-211 transition, marginally observed in comet P/Brorsen-Metcalf, was firmly detected in May 1990 in comet Austin. Observations performed at offset positions suggest that the source of H2CO might be distributed. The H2CO abundance is on the order of 0.5% that of water for both comets, assuming a scalelength of 104 km at 1 AU from the Sun for the distributed source. During the May observing period of comet Austin, two new species were detected for the first time in a comet: hydrogen sulfide H2S through its 110-101 ortho line at 169 GHz, and methanol CH3OH through J(3-2) ΔK = 0 transitions at 145 GHz. Preliminary estimates of their abundances are 1.5 10-3 for H2S and 8 10-3 for CH3OH.