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.

中文翻译：

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彗星 C/2016 R2 (Pan-STARRS) 中的离子发射

我们使用安装在印度阿布山红外天文台 1.2 米望远镜上的低分辨率光谱仪对奇特的彗星 C/2016 R2 (Pan-STARRS) 进行了观测。这颗彗星在 2018 年 1 月的两个日期被观测到，当时它的日心距离为 2.8 天文单位。基于我们观察的研究表明，与一般彗星光谱相比，这颗彗星的光谱非常不寻常。C/2016 R2 彗星中没有大多数主要的彗星排放，如 C$_{2}$、C$_{3}$ 和 CN。然而，彗星光谱显示出来自诸如 CO$^{+}$ 和 N$_{2}^{+}$ 等离子物质的非常强的发射带。0.09 $\pm$ 0.02 的平均 N$_{2}$/CO 比率是从光谱中得出的，并且与太阳星云相比，对于该比率估计了 1.6 $\pm$ 0.4 的极低损耗因子。我们还检测到超过 5400 埃的次要发射特征，尽管很少。CO$^{+}$ 和 N$_{2}^{+}$ 的柱密度由它们的发射带计算。光谱表明彗星冰以 CO 为主。与太阳星云相比，这颗彗星中 N$_{2}$/CO 比率的低消耗因子和彗星的不寻常光谱是独特处理的结果在它在早期太阳星云中形成的位置。