Molecular composition of short-period comets from millimetre-wave spectroscopy: 21P/Giacobini-Zinner, 38P/Stephan-Oterma, 41P/Tuttle-Giacobini-Kresák, and 64P/Swift-Gehrels

International audienceWe present the results of millimetre-wave spectroscopic observations and spectral surveys of the following short-period comets: 21P/Giacobini-Zinner in September 2018, 41P/Tuttle-Giacobini-Kresák in April 2017, and 64P/Swift-Gehrels and 38P/Stephan-Oterma in December 2018, carried out with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m radio telescope at wavelengths between 1 and 3 mm. Comet 21P was also observed in November 1998 with the IRAM 30-m, James Clerk Maxwell Telescope, and the Caltech submillimeter Observatory radio telescopes at wavelengths from 0.8 to 3 mm. The abundances of the following molecules have been determined in those comets: HCN, CH3OH, CS, H2CO, CH3CN, and H2S in comet 21P; HCN and CH3OH in 41P; HCN, CH3OH, and CS in 64P; and CH3OH in 38P. The last three comets, classified as carbon-chain typical from visible spectro-photometry, are relatively rich in methanol (3.5–5% relative to water). On the other hand, comet 21P, classified as carbon-chain depleted, shows abundances relative to water which are low for methanol (1.7%), very low for H2S (0.1%), and also relatively low for H2CO (0.16%) and CO (