Forbidden atomic carbon, nitrogen, and oxygen emission lines in the water-poor comet C/2016 R2 (Pan-STARRS)

Context. The N[SUB]2[/SUB] and CO-rich and water-depleted comet C/2016 R2 (Pan-STARRS) - hereafter "C/2016 R2" - is a unique comet for detailed spectroscopic analysis. <BR /> Aims: We aim to explore the associated photochemistry of parent species, which produces different metastable states and forbidden emissions, in this cometary coma of peculiar composition. <BR /> Methods: We reanalyzed the high-resolution spectra of comet C/2016 R2 obtained in February 2018 using the UVES spectrograph of the European Southern Observatory Very Large Telescope. Various forbidden atomic emission lines of [CI], [NI], and [OI] were observed in the optical spectrum of this comet when it was at 2.8 au from the Sun. The observed forbidden emission intensity ratios are studied in the framework of a couple-chemistry emission model. <BR /> Results: The model calculations show that CO[SUB]2[/SUB] is the major source of both atomic oxygen green and red doublet emissions in the coma of C/2016 R2 (while for most comets it is generally H[SUB]2[/SUB]O), whereas, CO and N[SUB]2[/SUB] govern the atomic carbon and nitrogen emissions, respectively. Our modeled oxygen green-to-red-doublet and carbon-to- nitrogen emission ratios are higher by a factor of three than what is found from observations. These discrepancies could be due to uncertainties associated with photon cross sections or unknown production and/or loss sources. Our modeled oxygen green-to-red-doublet emission ratio is close to what is seen in observations when we consider an O[SUB]2[/SUB] abundance with a production rate of 30% relative to the CO production rate. We constrained the mean photodissociation yield of CO, producing C([SUP]1[/SUP]S) at about 1%, a quantity which has not been measured in the laboratory. The collisional quenching is not a significant loss process for N([SUP]2[/SUP]D) though its radiative lifetime is significant (~10 h). Hence, the observed [NI] doublet- emission ratio ([NI] 5198/5200) of 1.22, which is smaller than the terrestrial measurement by a factor 1.4, is mainly due to the characteristic radiative decay of N([SUP]2[/SUP]D).