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 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 H _ { 2 } O .
A coupled chemistry-emission model is developed to study the production and loss mechanisms of 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 geocentric distance of the comet .
Calculations show that the contribution of photodissociation of H _ { 2 } O to the green ( red ) line emission is 30 to 70 % ( 60 to 90 % ) , while CO _ { 2 } and CO are the next potential sources contributing 25 to 50 % ( < 5 % ) .
The ratio of the photo-production rate of O ( ^ { 1 } S ) to O ( ^ { 1 } D ) would be around 0.03 ( \pm 0.01 ) if H _ { 2 } O is the main source of oxygen lines , whereas it is \sim 0.6 if the parent is CO _ { 2 } .
Our calculations suggest that the yield of O ( ^ { 1 } S ) production in the photodissociation of H _ { 2 } O can not 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 .