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 Millimetrique, and the Nancay radio telescope. We report on observations of OH, HCN, CH3OH, H2CO, 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 × 1029 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 CH3OH (4%) and H2CO (1%) exhibit some of the largest abundances compared with previous comets.