Coupled Chemistry-Emission Model for Atomic Oxygen Green and Red-doublet Emissions in Comet C/1996 B2 Hyakutake

Anil Bhardwaj & Susarla Raghuram (Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, India)

The green (5577 Å) and red-doublet (6300, 6364 Å) lines are prompt emissions of metastable oxygen atoms in the 1S and 1D states, respectively, that have been observed in several comets. The value of intensity ratio of green to red-doublet (G/R ratio) of 0.1 has been used as a benchmark to identify the parent molecule of oxygen lines as H2O. A coupled chemistry-emission model is developed to study the production and loss mechanisms of O(1S) and O(1D) atoms and the generation of red and green lines in the coma of C/1996 B2 Hyakutake.
The G/R ratio depends not only on photochemistry but also on the projected area observed for cometary coma, which is a function of the dimension of the slit used and geocentric distance of the comet. Calculations show that the contribution of photodissociation of H2O to the green (red) line emission is 30 to 70% (60 to 90%), while CO2 and CO are the next potential sources contributing 25 to 50% (< 5%). The ratio of the photo-production rate of O(1S) to O(1D) would be around 0.03 (± 0.01) if H2O is the main source of oxygen lines, whereas it is ~0.6 if the parent is CO2. Our calculations suggest that the yield of O(1S) production in the photodissociation of H2O cannot be larger than 1%. The model calculated radial brightness profiles of the red and green lines and G/R ratios are in good agreement with the observations made on comet Hyakutake in March 1996.

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