4 short-period comets spectra

We 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-Kresak in April 2017, 64P/Swift-Gehrels and 38P/Stephan-Oterma 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, CH_3_OH, CS, H_2_CO, CH_3_CN and H_2_S in comet 21P, HCN and CH_3_OH in 41P, HCN, CH_3_OH and CS in 64P, and CH_3_OH in 38P. The three last 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%).