Abstract :
[en] The returning long period comet C/2016 R2 (PanSTARRS) was discovered on September 7, 2016 at 6.3 au from the Sun. While it was already showing a 20" coma at this large distance (Weryk and Wainscoat 2016), it is only in December 2017 that it was found that this comet had a very unusual composition. From radio observations the comet appeared to be very rich in CO and very poor in HCN (Wierzchos and Womack 2018) and its optical spectrum was dominated by CO[SUP]+[/SUP] and more surprisingly N[SUB]2[/SUB][SUP]+[/SUP] emission bands (Cochran and McKay 2018), while most of the emission bands usually detected in the optical spectrum of comets were not detected. In order to investigate in detail its coma in the optical, we obtained a total of 6 hours of Director Discretionary Time on C/2016 R2 with UVES, the high resolution optical spectrograph of the ESO Very Large Telescope, between February 11 and 16, 2018. We used two different settings to optimally cover the whole optical spectrum (326-1060 nm) with a resolving power of 80.000. We report on those observations. We detect strong emissions of the ions CO[SUP]+[/SUP] and N[SUB]2[/SUB][SUP]+[/SUP], and also several CO[SUB]2[/SUB][SUP]+ [/SUP]bands, but no H[SUB]2[/SUB]O[SUP]+[/SUP] . We detect emission lines of the radicals CN, C[SUB]2[/SUB] and C[SUB]3[/SUB] but they are very weak. We computed from these spectra the N[SUB]2[/SUB][SUP]+[/SUP] / CO[SUP]+[/SUP] / CO[SUB]2[/SUB][SUP]+[/SUP] ratios in the coma of the comet which put some constraints on the comet formation models, and compared those values to other comets. The forbidden oxygen [OI] lines are detected, allowing to measure the ratio between the green line and the red doublet which provides a way to determine the abundance of CO and CO[SUB]2[/SUB] relative to H[SUB]2[/SUB]O. For the first time we report the detection of the nitrogen [NI] forbidden doublet at 5197.9 and 5200.2 Å in the coma of a comet, confirming the high abundance of nitrogen in this comet. Interestingly we also detect a line at 9850 Å which could be one of the carbon [CI] forbidden lines but we do not detect the other line of the doublet at 9823 Å. Because of the strong N[SUB]2[/SUB][SUP]+[/SUP] emissions, it was also a unique opportunity to measure the [SUP]14[/SUP]N/[SUP]15[/SUP]N isotopic ratio directly in N[SUB]2[/SUB], the main nitrogen reservoir in the solar nebula.
