Chemical composition diversity among 24 comets observed at radio wavelengths

We present a comparative study on molecular abundances in comets based on millimetre/submillimetre observations made with the IRAM 30-m, JCMT, CSO and SEST telescopes. This study concerns a sample of 24 comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observed from 1986 to 2001 and 8 molecular species (HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, H2S). HCN was detected in all comets, while at least 2 molecules were detected in 19 comets. From the sub-sample of comets for which contemporary H2O production rates are available, we infer that the HCN abundance relative to water varies from 0.08% to 0.25%. With respect to other species, HCN is the molecule which exhibits the lowest abundance variation from comet to comet. Therefore, production rates relative to that of HCN can be used for a comparative study of molecular abundances in the 19 comets. It is found that: CH3OH/HCN varies from less than or equal to9 to 64; CO/HCN varies from less than or equal to24 to 180; H2CO/HCN varies between 1.6 and 10; and H2S/HCN varies between 1.5 and 7.6. This study does not show any clear correlation between the relative abundances and the dynamical origins of the comets, or their dust-to-gas ratios.