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

N Biver,G Paubert,A Mckay,R Moreno,D Bockelée-Morvan,J Boissier,M A Disanti,F Kugel,E Bertrand,N Dello Russo,J Crovisier,H Boussier

doi:10.1051/0004-6361/201833449

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.

Highlights

Comets are the most pristine remnants of the formation of the solar system 4.6 billion yr ago
We report here observations of a very peculiar comet, C/2016 R2 (PanSTARRS), with the 30-m telescope of the Institut de radioastronomie millimétrique (IRAM)
We present millimetre observations of CO, CH3OH, H2CO, HCN, CO+ and upper limits on several other molecules obtained with the IRAM 30-m, complemented by observations of the OH radical with the Nançay radio telescope, and amamteuilrlimobesterervdaattiao,nws eodfeCriOve+c, oNns+2traanindtsdounstthceognatisntueummp.erFartoumre the and outgassing pattern

Summary

Introduction

Comets are the most pristine remnants of the formation of the solar system 4.6 billion yr ago. We report here observations of a very peculiar comet, C/2016 R2 (PanSTARRS), with the 30-m telescope of the Institut de radioastronomie millimétrique (IRAM) This object is a long-period, dynamically old, Oort cloud comet that passed close to the Sun 21 600 yr ago and has an orbit inclination of 58◦. We present millimetre observations of CO, CH3OH, H2CO, HCN, CO+ and upper limits on several other molecules obtained with the IRAM 30-m, complemented by observations of the OH radical with the Nançay radio telescope, and amamteuilrlimobesterervdaattiao,nws eodfeCriOve+c, oNns+2traanindtsdounstthceognatisntueummp.erFartoumre the and outgassing pattern Production rates and their time evolution on 23 and 24 January, as well as abundances relative to CH3OH or CO are derived.

Observation of OH radical with the Nançay radio telescope

Analysis and results

Findings

Discussion

Summary and conclusion