Forbidden oxygen lines in comets at various heliocentric distances^⋆

Institut d’Astrophysique et Géophysique et Océanographie, Université de Liège, Allée du 6 août 17, 4000 Liège, Belgium
e-mail: adecock@ulg.ac.be

Received: 19 September 2012
Accepted: 12 April 2013

Abstract

We present a study of the three forbidden oxygen lines [OI] located in the optical region – i.e., 5577.339  Å (the green line), 6300.304  Å and 6363.776  Å (the two red lines) – in order to better understand the production of these atoms in cometary atmospheres. The analysis is based on 48 high-resolution and high signal-to-noise spectra collected with UVES at the ESO VLT between 2003 and 2011 referring to 12 comets of different origins observed at various heliocentric distances. The flux ratio of the green line to the sum of the two red lines is evaluated to determine the parent species of the oxygen atoms by comparison with theoretical models. This analysis confirms that, at about 1 AU, H₂O is the main parent molecule producing oxygen atoms. At heliocentric distances >2.5 AU, this ratio changes rapidly, an indication that other molecules are starting to contribute. The most abundant species after H₂O in the coma, CO and CO₂, are good candidates, and the ratio is used to estimate their abundances. We found that the CO₂ abundance relative to H₂O in comet C/2001 Q4 (NEAT) observed at 4 AU can be as high as ~70%. The intrinsic widths of the oxygen lines were also measured. The green line is on average about 1 km s^-1 broader than the red lines, while the theory predicts that the red lines are broader. This might be due to the nature of the excitation source or to a contribution of CO₂ as the parent molecule of the 5577.339  Å line. At 4 AU, we found that the width of the green and red lines in comet C/2001 Q4 are the same, which could be explained if CO₂ becomes the main contributor to the three [OI] lines at high heliocentric distances.