Wednesday, October 3, 2012

1210.0842 (A. Decock et al.)

Forbidden oxygen lines in comets at various heliocentric distances    [PDF]

A. Decock, E. Jehin, D. Hutsemékers, J. Manfroid
We present a study of the three forbidden oxygen lines [OI] located in the optical region (i.e., 5577.339 {\AA}, 6300.304 {\AA} and 6363.776 {\AA}) 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 ratios I_6300/I_6364 and G/R=I_5577/(I_6300+I_6364) are evaluated to determine the parent species of the oxygen atoms by comparison with theoretical models. This analysis confirms that, at around 1 AU, H2O is the main parent molecule producing oxygen atoms. At heliocentric distances > 2.5 AU, the G/R 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 G/R ratio is used to estimate their abundances. We found that the CO2 abundance relative to H2O in comet Q4 (NEAT) observed at 4 AU could be as high as ~ 70%. The intrinsic widths of the oxygen lines were also measured. The green line is in average about 1 km/s broader than the red lines while the theory predicts the red lines should be broader. But at 4 AU, we found that the width of the green and red lines in comet Q4 (NEAT) are the same which could be explained if CO2 is the main contributor.
View original: http://arxiv.org/abs/1210.0842

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