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.