Radio wavelength molecular observations of comets C/1999 T1 (McNaught-Hartley), C/2001 A2 (LINEAR), C/2000 WM₁ (LINEAR) and 153P/Ikeya-Zhang

¹ LESIA, CNRS UMR 8109, Observatoire de Paris, 5 pl. J. Janssen, 92195 Meudon, France e-mail: Nicolas.Biver@obspm.fr
² California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA
³ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
⁴ Observatoire de Bordeaux, BP 89, 33270 Floirac, France
⁵ IRAM, Avd. Divina Pastora, 7, 18012 Granada, Spain
⁶ St. Cloud State University, MS 324, St. Cloud, MN 56301-4498, USA

Received: 18 July 2005
Accepted: 7 December 2005

Abstract

We present a comparative study of the relative abundances of CO, CH₃OH, H₂CO, HCN, HNC, CS, H₂S, CH₃CN, SO and HNCO in comets C/1999 T1 (McNaught-Hartley), C/2001 A2 (LINEAR), C/2000 WM₁ (LINEAR) and 153P/Ikeya-Zhang, four of the brightest comets seen in 2001–2002. This investigation is based on millimetre/submillimetre observations made with the IRAM 30-m, SEST, CSO and Kitt Peak 12-m telescopes. Although these four comets are expected to originate from the Oort cloud, they present significant differences in molecular abundances, especially as regards to the most volatile species: CO and H₂S. In particular comet C/2000 WM₁ looks quite depleted in these volatiles, suggesting it may have a different origin than the others. Heliocentric variations of molecular relative abundance in the coma are also investigated. Significant increases in the CS/HCN and HNC/HCN production rate ratios with decreasing heliocentric distances are observed.