A coupled chemistry-emission model for atomic oxygen green and red-doublet emissions in the comet C/1996 B2 Hyakutake

Abstract

The green (5577 Å) and red-doublet (6300, 6364 Å) lines are prompt emissions of metastable oxygen atoms in the ¹ S and ¹ D states, respectively, that have been observed in several comets. The value of the 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 the O(¹ S) and O(¹ D) 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 the geocentric distance of the comet. Calculations show that the contribution of photodissociation of H₂O to the green (red) line emission is 30%-70% (60%-90%), while CO₂ and CO are the next potential sources contributing 25%-50% (<5%). The ratio of the photoproduction rate of O(¹ S) to O(¹ D) 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 the comet Hyakutake in 1996 March.