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The recent observations show that comet C/2016 R2 (Pan-Starrs) has a uniqueand peculiar composition when compared with several other comets observed at2.8 au heliocentric distance. Assuming solar resonance fluorescence is the onlyexcitation source, the observed ionic emission intensity ratios are used toconstrain the corresponding neutral abundances in this comet. We developed aphysico-chemical model to study the ion density distribution in the inner comaof this comet by accounting for photon and electron impact ionization ofneutrals, charge exchange and proton transfer reactions between ions andneutrals, and electron-ion thermal recombination reactions. Our calculationsshow that CO2+ and CO+ are the major ions in the inner coma, and close to thesurface of nucleus CH3OH+, CH3OH2+ and O2+ are also important ions. Byconsidering various excitation sources, we also studied the emission mechanismsof different excited states of CO+, CO2+, N2+, and H2O+. We found that thephoton and electron impact ionization and excitation of corresponding neutralssignificantly contribute to the observed ionic emissions for radial distancessmaller than 300 km and at larger distances, solar resonance fluorescence isthe major excitation source. Our modelled ion emission intensity ratios areconsistent with the ground-based observations. Based on the modelled emissionprocesses, we suggest that the observed ion emission intensity ratios can beused to derive the neutral composition in the cometary coma only when the iondensities are significantly controlled by photon and photoelectron impactionization of neutrals rather than by the ion-neutral chemistry.

A physico-chemical model to study the ion densitydistribution in the inner coma of comet C/2016 R2(Pan-STARRS)