Forbidden oxygen lines at various nucleocentric distances in comets

Aims. We study the formation of the [OI] lines – that is, 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (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, I5577/(I6300 + I6364), (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 (