The extraordinary composition of the blue comet C/2016 R2 (PanSTARRS)^★,★★

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: nicolas.biver@obspm.fr
² IRAM, Avd. Divina Pastora, 7, 18012 Granada, Spain
³ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁴ Commission des comètes, Société Astronomique de France, 3 rue Beethoven, 75016 Paris, France
⁵ Observatoire de Dauban, 04 Banon, France
⁶ NASA GSFC/USRA, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
⁷ Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
⁸ NASA Goddard Center for Astrobiology, NASA GSFC, Mail Stop 690, Greenbelt, MD 20771, USA

Received: 18 May 2018
Accepted: 12 September 2018

Abstract

We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). This comet was observed on 23 and 24 January 2018 with the IRAM 30 m telescope, and in January to March 2018 with the Nançay radio telescope. Visible spectroscopy was performed in December 2017 and February 2018 with small amateur telescopes. We report on measurements of CO, CH₃OH, H₂CO and HCN production rates, and on the determination of the N₂/CO abundance ratio. Several other species, especially OH, were searched for but not detected. The inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of ~2.8 AU. The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N₂ and CO and dust poor. This suggests that this comet might belong to a very rare group of comets formed beyond the N₂ ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers.