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

We present the results of millimetre-wave spectroscopic observations and spectral surveys of the following short-period comets: 41P/Tuttle-Giacobini-Kresák in April 2017, 21P/Giacobini-Zinner in September 2018, 38P/Stephan-Oterma and 64P/Swift-Gehrels in December 2018, carried out with the 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, JCMT and CSO radio telescopes at wavelengths from 0.8 to 3 mm. The abundances of the following molecules have been determined in those comets: HCN and CH3OH in 41P, CH3OH in 38P, HCN, CH3OH and CS in 64P, and HCN, CH3OH, CS, H2CO, CH3CN and H2S in comet 21P. The first three comets, classified as carbon-chain typical from visible spectro-photometry, are relatively rich in methanol (3.5 to 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 (<2.5%). Observations of comet 21P do not show any change in activity and composition between the 1998 and 2018 perihelions. Sensitive upper limits on the abundances of other molecules of interest are also reported for these comets.