Forbidden oxygen lines at various nucleocentric distances in comets

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 determine the parent species of the oxygen atoms using the green to red-doublet emission intensity ratio (G/R ratio) and the lines velocity widths. We acquired at the ESO VLT high-resolution spectroscopic observations of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 when they were close to the Earth (< 0.6 au). Using the observed spectra, we determined the intensities and the widths of the three [OI] lines. We have spatially extracted the spectra in order to achieve the best possible resolution of about 1-2, i.e., nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We have decontaminated the [OI] green line from C2 lines blends. It is found that the observed G/R ratio on all four comets varies as a function of nucleocentric projected distance. 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 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 seems to produce O(1S) closer to the nucleus while the water molecule forms all the O(1S) and O(1D) atoms beyond 1000 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 coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upper limits 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 CO2 relative abundance in comets.

Forbidden oxygen lines in comets at various heliocentric distances

We present a study of the three forbidden oxygen lines [OI] located in the optical region (i.e., 5577.339 r{A} (the green line), 6300.304 r{A} and 6363.776 r{A} (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 is changing rapidly, an indication that other molecules are starting to contribute. CO and CO2, the most abundant species after H2O in the coma, 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 broader than the red lines while the theory predicts the red lines to be broader. This might be due to the nature of the excitation source and/or a contribution of CO2 as parent molecule of the 5577.339 r{A} 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 for the three [OI] lines at high heliocentric distances.