C/2016 R2 (PANSTARRS): A comet rich in CO and depleted in HCN

Abstract

We observed comet C/2016 R2 (PANSTARRS) with the ARO 10-m SMT, and report the first detection of CO emission from this comet with amounts high enough to be the primary driver of activity. We obtained spectra and maps of the CO J=2-1 rotational line at 230 GHz between 2017 December and 2018 January. We calculated an average production rate of Q(CO)=(4.6+/-0.4)x10${}^{28}$ mol s${}^{-1}$ at r ~2.9 au and delta ~2.1 au. The CO line is thin FWHM ~ 0.8 km s${}^{-1}$ with a slight blue-shift ~ -0.1 km s${}^{-1}$ from the ephemeris velocity, and we derive a gas expansion velocity of V${}_{exp}$ = 0.50+/-0.15 km s${}^{-1}$. This comet produced approximately half the CO that comet C/1995 O1 (Hale-Bopp) did at 3 au. If CO production scales with nucleus surface area, then the radius need not exceed ~15 km. The spectra and mapping data are consistent with CO arising from a combination of a sunward-side active area and an isotropic source. For HCN, we calculated a 3-sigma upper limit production rate of Q(HCN) < 8x10${}^{24}$ molecules s${}^{-1}$, which corresponds to an extraordinarily high abundance ratio limit of Q(CO)/Q(HCN) > 5000. We inferred a production rate of molecular nitrogen of Q(N${}_2$) ~2.8x10${}^{27}$ molecules s${}^{-1}$ using our CO data and the reported N${}_2$/CO column density ratio (Cochran & MacKay 2018a,b). The comet does not show the typical nitrogen depletion seen in comets, and the CO-rich, N${}_2$-rich and HCN-depleted values are consistent with formation in a cold environment of T < 50 K that may have provided significant N${}_2$ shielding.

arXiv comment14 pages, 2 figures, 2 tables. Accepted for publication in The Astronomical Jorunal

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Cited by in corpus (13)

• Unusually High CO Abundance of the First Active Interstellar Comet
• The Peculiar Volatile Composition of CO-Dominated Comet C/2016 R2 (PanSTARRS)
• The extraordinary composition of the "blue comet" C/2016 R2 (PanSTARRS)
• High resolution optical spectroscopy of the ${\mathrm{N}}_2$-rich comet C/2016 R2 (PanSTARRS)
• CO gas and dust outbursts from Centaur 29P/Schwassmann-Wachmann
• A SUBLIME 3D Model for Cometary Coma Emission: the Hypervolatile-Rich Comet C/2016 R2 (PanSTARRS)
• Atomic carbon, nitrogen, and oxygen forbidden emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)
• Global Modeling of Nebulae With Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles
• A physico-chemical model to study the ion densitydistribution in the inner coma of comet C/2016 R2(Pan-STARRS)
• Ionic emissions in comet C/2016 R2 (Pan-STARRS)
• C/O ratios in self-gravitating protoplanetary discs with dust evolution
• The N${}_2$ Production Rate in Comet C/2016 R2 (PanSTARRS)
• On The Spin Dynamics of Elongated Minor Bodies with Applications to a Possible Solar System Analogue Composition for `Oumuamua