SIMBAD references

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.