A SUBLIME 3D Model for Cometary Coma Emission: the Hypervolatile-Rich
  Comet C/2016 R2 (PanSTARRS)
  The coma of comet C/2016 R2 (PanSTARRS) is one of the most chemically
peculiar ever observed, in particular due to its extremely high CO/H2O and
N2+/H2O ratios}, and unusual trace volatile abundances. However, the complex
shape of its CO emission lines, as well as uncertainties in the coma structure
and excitation, has lead to ambiguities in the total CO production rate. We
performed high resolution, spatially, spectrally and temporally resolved CO
observations using the James Clerk Maxwell Telescope (JCMT) and Submillimeter
Array (SMA) to elucidate the outgassing behaviour of C/2016 R2. Results are
analyzed using a new, time-dependent, three dimensional radiative transfer code
(SUBLIME), incorporating for the first time, accurate state-to-state
collisional rate coefficients for the CO--CO system. The total CO production
rate was found to be in the range $(3.8-7.6)\times10^{28}$ s$^{-1}$ between
2018-01-13 and 2018-02-01, with a mean value of $(5.3\pm0.6)\times10^{28}$
s$^{-1}$ at r_H = 2.8-2.9 au. The emission is concentrated in a near-sunward
jet, with an outflow velocity $0.51\pm0.01$ km/s, compared to $0.25\pm0.01$
km/s in the ambient (and night-side) coma. Evidence was also found for an
extended source of CO emission, possibly due to icy grain sublimation around
$1.2\times10^5$ km from the nucleus. Based on the coma molecular abundances, we
propose that the nucleus ices of C/2016 R2 can be divided into a rapidly
sublimating apolar phase, rich in CO, CO2, N2 and CH3OH, and a predominantly
frozen (or less abundant), polar phase containing more H2O, CH4, H2CO and HCN.
