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Ionic emissions in comet C/2016 R2 (Pan-STARRS)

We carried out observations of a peculiar comet, C/2016 R2 (Pan-STARRS), using a low resolution spectrograph mounted on the 1.2m telescope at Mount Abu Infrared Observatory, India. The comet was observed on two dates in January 2018, when it was at a heliocentric distance of 2.8 AU. Study based on our observations revealed that the optical spectrum of this comet is quite unusual as compared to general cometary spectra. Most of the major cometary emissions like C$_{2}$, C$_{3}$ and CN were absent in comet C/2016 R2. However, the comet spectrum showed very strong emission bands from ionic species like CO$^{+}$ and N$_{2}^{+}$. A mean N$_{2}$/CO ratio of 0.09 $\pm$ 0.02 was derived from the spectra and an extremely low depletion factor of 1.6 $\pm$ 0.4 has been estimated for this ratio as compared to the solar nebula. We have also detected minor emission features beyond 5400 Angstroms, albeit marginally. The column densities of CO$^{+}$ and N$_{2}^{+}$ were calculated from their emission bands. The optical spectrum suggests that the cometary ice is dominated by CO. The low depletion factor of N$_{2}$/CO ratio in this comet, as compared to 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.