2020MNRAS.495.3559V

We have carried out observations of a peculiar comet, C/2016 R2 (Pan-STARRS), using a low-resolution spectrograph mounted on the 1.2-m telescope at Mount Abu Infrared Observatory, India. The comet was observed on two dates in 2018 January, when it was at a heliocentric distance of 2.8 au. A study based on our observations has revealed that the optical spectrum of this comet is quite unusual compared with general cometary spectra. Most major cometary emissions, such as C₂, C₃ and CN, were absent in comet C/2016 R2. However, the comet's spectrum showed very strong emission bands from ionic species such as CO⁺ and N₂⁺. A mean N₂/CO ratio of 0.09 ± 0.02 was derived from the spectra and an extremely low depletion factor of 1.6 ± 0.4 has been estimated for this ratio compared with the solar nebula. We have also detected minor emission features beyond 5400 Å, albeit marginally. The column densities of CO⁺ and N₂⁺ were calculated from their emission bands. The optical spectrum suggests that the cometary ice is dominated by CO. The low depletion factor of the N₂/CO ratio in this comet, compared with the solar nebula, and the unusual spectrum of the comet are consequences of distinctive processing at the location of its formation in the early solar nebula.