Spectroscopic Observations of Comet C/1999 H1 (Lee) with the SEST, JCMT, CSO, IRAM, and NanÇay Radio Telescopes

Abstract

Coordinated spectroscopic radio observations of comet C/1999 H1 (Lee) were undertaken between 1999 May 4 and 1999 October 26, using the Swedish-ESO Submillimetre Telescope, the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory, the 30 m telescope of the Institut de Radio Astronomie Millimétrique, and the Nançay radio telescope. We report on observations of OH, HCN, CH 3OH, H 2CO, and CS and on the evolution of their production rates with heliocentric distance between 0.8 and 1.7 AU, where the total outgassing rate ranged between 0.2 and 1.6 × 10 29 molecules s -1. HNC was detected unexpectedly in this medium-activity comet with a relatively large HNC/HCN mixing ratio of 12%, close to that measured in comet C/1995 O1 (Hale-Bopp), which cannot be explained by current chemical models of the coma. CO was tentatively detected with a low abundance around 4% relative to water and is clearly underabundant in comparison to that in comets Hyakutake and Hale-Bopp. An upper limit of D/H less than 300 × 10 -5 in water was found from a brief search for HDO. Molecular abundances relative to water of the other species around 1 AU are similar to those observed in other comets, although CH 3OH (4%) and H 2CO (1%) exhibit some of the largest abundances compared with previous comets.