Water, hydrogen cyanide, carbon monoxide, and dust production from distant comet 29P/Schwassmann-Wachmann 1^★,★★

¹ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 5 place Jules Janssen, 92195 Meudon, France
e-mail: dominique.bockelee@obspm.fr
² Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1, Orlando FL 32826, USA
³ Department of Physics, University of Central Florida, Orlando, FL 32816 USA
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁵ Astrochemistry Laboratory, Goddard Space Flight Center, NASA, 8800 Greenbelt Rd., Greenbelt, MD 20771 USA
⁶ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, Bus-2410, 3000 Belgium
⁸ Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Liège, Allée du 6 Août 17, 4000 Belgium
⁹ European Space Agency European Space Astronomy Centre, Camino Bajo el Castillo, s/n Urbanización Villafranca del Castillo 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 USA
¹¹ Centrum Badań Kosmicznych Polskiej Akademii Nauk (CBK PAN), Bartycka 18A, Warszawa 00-716, Poland
¹² INAF – Istituto di Astrofísica e Planetologia Spaziali, Area Ricerca Tor Vergata, Via Fosso del Cavaliere 100, 00133 Rome, Italy

Received: 1 February 2022
Accepted: 14 May 2022

Abstract

Context. 29P/Schwassmann-Wachmann 1 is a distant Centaur/comet, showing persistent CO-driven activity and frequent outbursts.

Aims. We aim to better characterize its gas and dust activity from multiwavelength observations performed during outbursting and quiescent states.

Methods. We used the HIFI, PACS and SPIRE instruments of the Herschel space observatory on several dates in 2010, 2011, and 2013 to observe the H₂O 557 GHz and NH₃ 573 GHz lines and to image the dust coma in the far-infrared. Observations with the IRAM 30 m telescope were undertaken in 2007, 2010, 2011, and 2021 to monitor the CO production rate through the 230 GHz line, and to search for HCN at 89 GHz. The 70 and 160 µm PACS images were used to measure the thermal flux from the nucleus and the dust coma. Modeling was performed to constrain the size of the sublimating icy grains and to derive the dust production rate.

Results. HCN is detected for the first time in comet 29P (at 5σ in the line area). H₂O is detected as well, but not NH₃. H₂O and HCN line shapes differ strongly from the CO line shape, indicating that these two species are released from icy grains. CO production rates are in the range (2.9–5.6) × 10²⁸ s⁻¹ (1400–2600 kg s⁻¹). A correlation between the CO production rate and coma brightness is observed, as is a correlation between CO and H₂O production. The correlation obtained between the excess of CO production and excess of dust brightness with respect to the quiescent state is similar to that established for the continuous activity of comet Hale-Bopp. The measured Q(H₂O)/Q(CO) and Q(HCN)/Q(CO) production rate ratios are 10.0 ± 1.5 % and 0.12 ± 0.03 %, respectively, averaging the April-May 2010 measurements (Q(H₂O) = (4.1 ± 0.6) × 10²⁷ s⁻¹, Q(HCN) = (4.8 ± 1.1) × 10²⁵ s⁻¹). We derive three independent and similar values of the effective radius of the nucleus, ~31 ± 3 km, suggesting an approximately spherical shape. The inferred dust mass-loss rates during quiescent phases are in the range 30–120 kg s⁻¹, indicating a dust-to-gas mass ratio <0.1 during quiescent activity. We conclude that strong local heterogeneities exist on the surface of 29P, with quenched dust activity from most of the surface, but not in outbursting regions.

Conclusions. The volatile composition of the atmosphere of 29P strongly differs from that of comets observed within 3 au from the Sun. The observed correlation between CO, H₂O and dust activity may provide important constraints for the outburst-triggering mechanism.