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To study the formation of the [OI] lines - i.e., 5577 A (the green line),6300 A and 6364 A (the two red lines) - in the coma of comets and to determinethe parent species of the oxygen atoms using the green to red-doublet emissionintensity ratio (G/R ratio) and the lines velocity widths. We acquired at theESO VLT high-resolution spectroscopic observations of comets C/2002 T7(LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 whenthey were close to the Earth (< 0.6 au). Using the observed spectra, wedetermined the intensities and the widths of the three [OI] lines. We havespatially extracted the spectra in order to achieve the best possibleresolution of about 1-2", i.e., nucleocentric projected distances of 100 to 400km depending on the geocentric distance of the comet. We have decontaminatedthe [OI] green line from C2 lines blends. It is found that the observed G/Rratio on all four comets varies as a function of nucleocentric projecteddistance. This is mainly due to the collisional quenching of O(1S) and O(1D) bywater molecules in the inner coma. The observed green emission line width isabout 2.5 km/s and decreases as the distance from the nucleus increases whichcan be explained by the varying contribution of CO2 to the O(1S) production inthe innermost coma. The photodissociation of CO2 molecules seems to produceO(1S) closer to the nucleus while the water molecule forms all the O(1S) andO(1D) atoms beyond 1000 km. Thus we conclude that the main parent speciesproducing O(1S) and O(1D) in the inner coma is not always the same. Theobservations have been interpreted in the framework of thecoupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upperlimits of CO2 relative abundances are derived from the observed G/R ratios.Measuring the [OI] lines could indeed provide a new way to determine the CO2relative abundance in comets.

Forbidden oxygen lines at various nucleocentric distances in comets