C/2016 R2 (PANSTARRS): A Comet Rich in CO and Depleted in HCN

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) × 1028 mol s−1 at r ∼ 2.9 au and Δ ∼ 2.1 au. The CO line is thin (ΔV FWHM ∼ 0.8 km s−1) with a slight blueshift (δv ∼ −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 R R2 ∼ 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 3σ upper limit production rate of Q(HCN) < 8 × 1024 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(N2) ∼ 2.8 × 1027 molecules s−1 using our CO data and the reported N2/CO column density ratio. The comet does not show the typical nitrogen depletion seen in comets. The CO-rich, N2-rich, and HCN-depleted values are consistent with formation in an environment of T ∼ 50 K that may have provided significant N2 shielding.