Forbidden oxygen lines at various nucleocentric distances in comets

Aims. We study the formation of the [OI] lines - that is, 5577.339 angstrom (the green line), 6300.304 angstrom and 6363.776 angstrom (the two red lines) - in the coma of comets and determine the parent species of the oxygen atoms using the ratio of the green-to-red-doublet emission intensity, I-5577/(I-6300 + I-6364), (hereafter the G/R ratio) and the line velocity widths. Methods. We acquired high-resolution spectroscopic observations at the ESO Very Large Telescope of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and 103P/Hartley 2 when they were close to Earth (<0.6 au). Using the observed spectra, which have a high spatial resolution (<60 km/pixel), we determined the intensities and widths of the three [OI] lines. We spatially extracted the spectra to achieve the best possible resolution of about 1-2 `', that is, nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We decontaminated the [OI] green line from C-2 lines blends that we identified. Results. The observed G/R ratio in all four comets varies as a function of nucleocentric projected distance (between similar to 0.25 to similar to 0.05 within 1000 km). This is mainly due to the collisional quenching of O(S-1) and O(D-1) by water molecules in the inner coma. The observed green emission line width is about 2.5 km s(-1) and decreases as the distance from the nucleus increases, which can be explained by the varying contribution of CO2 to the O(S-1) production in the innermost coma. The photodissociation of CO2 molecules seem to produce O(S-1) closer to the nucleus, while the water molecule forms all the O(S-1) and O(D-1) atoms beyond 10(3) km. Thus we conclude that the main parent species producing O(S-1) and O(D-1) in the inner coma is not always the same. The observations have been interpreted in the framework of the previously described coupled-chemistry-emission model, and the upper limits of the relative abundances of CO2 were derived from the observed G/R ratios. Measuring the [OI] lines might provide a new way to determine the CO2 relative abundance in comets.