Forbidden oxygen lines at various nucleocentric distances in comets^⋆,⋆⋆

¹ Institut d’Astrophysique, de Géophysique et Océanographie, Université de Liège, Allée du 6 août 17, 4000 Liège, Belgium
e-mail: adecock@ulg.ac.be
² University of Franche-Comté, Observatoire des Sciences de l’Univers THETA, Institut UTINAM – UMR CNRS 6213, BP 1615, 25010 Besançon Cedex, France
³ Space Physics Laboratory, Vikram Sarabhai Space Centre, 695 022 Trivandrum, India
⁴ Department of Physics, Imperial College London, London SW7 2AZ, UK

Received: 14 June 2014
Accepted: 19 September 2014

Abstract

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 (<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 C₂ lines blends that we identified.

Results. The observed G/R ratio in all four comets varies as a function of nucleocentric projected distance (between ~0.25 to ~0.05 within 1000 km). This is mainly due to the collisional quenching of O(¹S) and O(¹D) 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(¹S) production in the innermost coma. The photodissociation of CO₂ molecules seem to produce O(¹S) closer to the nucleus, while the water molecule forms all the O(¹S) and O(¹D) atoms beyond 10³ km. Thus we conclude that the main parent species producing O(¹S) and O(¹D) 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.