C/2016 R2 (PanSTARRS), a visitor from the Solar System’s distant Oort Cloud, sports a very complex tail dominated by blue light and is unusually rich in carbon monoxide and nitrogen, according to observations led by Paris Observatory astronomer Nicolas Biver.
Comet C/2016 R2 (PanSTARRS). Image credit: Damian Peach, www.damianpeach.com / Jose J. Chambo, www.cometografia.es.
C/2016 R2 is a dynamically old, Oort cloud comet with an estimated orbital period of 21,600 years.
Discovered on September 7, 2016, the comet has a highly eccentric orbit tilted at an angle of 58 degrees to the ecliptic, and a semi-major axis of 740 astronomical units (AU).
C/2016 R2 is very peculiar in the sense that in autumn 2017, while approaching the Sun, it exhibited a deep blue coma and tail.
In contrast, usual comets show a dust tail and a coma of neutral or yellowish color, resulting from the scattering of solar radiation by dust.
“Comets are the most pristine remnants of the formation of the Solar System 4.6 billion years ago,” said Dr. Biver and co-authors, who performed a multi-wavelength compositional study of C/2016 R2.
“Investigating the composition of cometary ices provides clues to the physical conditions and chemical processes at play in the primitive Solar Nebula.”
The astronomers observed C/2016 R2 on January 23-24, 2018 with the 30-m telescope of the Institut de radioastronomie millimetrique (IRAM), and in January to March 2018 with the Nancay radio telescope.
They detected the presence of carbon monoxide, water, methanol, formaldehyde, nitrogen, hydrogen cyanide, hydrogen sulfide and carbon monosulfide.
“The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N2 (nitrogen) and CO (carbon monoxide) and dust poor,” they noted.
“This suggests that this comet might belong to a very rare group of comets formed beyond the N2 ice line.”
“Alternatively, C/2016 R2 could be the fragment of a large and differentiated trans-Neptunian object, with properties characteristic of volatile-enriched layers.”
The team also found that the comet’s main body was losing about 5 tons of carbon monoxide each second.
“Comet C/2016 R2 is representative of a family of comets that we observe only rarely each century,” the scientists said.
“Besides C/1908 R1 (Morehouse) and C/1961 R1 (Humason), other candidates are comets 29P/Schwassmann-Wachmann 1 and C/2002 VQ94 (LINEAR) which both showed optical spectra dominated by strong emissions of CO+ and N+2 , characteristics of abundant CO and N2 production and high N2/CO ratio.”
“The diversity of the dust/gas ratios seen in these comets may favour the second scenario in which these comets are collisional fragments of differentiated trans-Neptunian objects.”
The findings will be published in the journal Astronomy & Astrophysics.
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N. Biver et al. 2018. The extraordinary composition of the ‘blue comet’ C/2016 R2 (PanSTARRS). A&A, in press; arXiv 1809.08086
