Modeling of the CO+ emission spectrum in comet C/2016 R2 (PanSTARRS)

The long period comet C/2016 R2 (PanSTARRS), discovered on 2016 September 7 by the PansSTARRS telescope, presented an unusual composition. Most of the emission lines due to radicals usually abundant in comets (such as C2, CN, C3 ) appeared to be faint while unexpected strong emission lines due to N2+ and CO+ ions were clearly apparent in the spectra obtained in the optical range (some fainter CO2+ emission lines were also present). It is the first comet with such bright N2+ emission lines but also the brightness and abundance of CO+ lines allow to study this ion in great details. We obtained in February 2018 high resolution spectra with the ESO 8-m Very Large Telescope and the UVES spectrograph, that allowed us to obtain a CO+ spectrum with an unprecedented signal-to-noise ratio. We also developed a fluorescence model for CO+ to model these spectra. We will present these observations, their modeling and a first estimate of the 12C/13C isotopic ratio for CO+ ions in comet C/2016 R2. It is the first measurement of such a ratio with ground-based telescopes, the only related measurement published so far being done in situ in the coma of comet 67P/Churyumov-Gerasimenko by the ROSINA instrument on-board the Rosetta spacecraft for CO and CO2 molecules.