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

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: nicolas.biver@obspm.fr
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³ Laboratoire d’astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁴ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁵ Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
⁶ Solar System Exploration Division, Astrochemistry Laboratory Code 691, NASA-GSFC, Greenbelt, MD 20771, USA
⁷ Department of Physics, Catholic University of America, Washington, DC 20064, USA
⁸ Department of Physics, American University, Washington, DC, USA
⁹ Solar System Exploration Division, Planetary Systems Laboratory Code 693, NASA-GSFC, Greenbelt, MD 20771, USA
¹⁰ UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK
¹¹ Koyoma Astronomical Observatory, Kyoto Sangyo University Motoyama, Kamingamo, Kita-ku, Kyoto 603-8555, Japan

Received: 9 March 2021
Accepted: 17 May 2021

Abstract

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-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 (<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 such as CO, HNCO, HNC, or SO are also reported for these comets.