Photochemistry of atomic oxygen green and red-doublet emissions in comets at larger heliocentric distances

Abstract

Context. In comets, the atomic oxygen green (5577 Å) to red-doublet (6300, 6364 Å) emission intensity ratio (G/R ratio) of 0.1 has been used to confirm H₂O as the parent species producing forbidden oxygen emission lines. The larger (>0.1) value of G/R ratio observed in a few comets is ascribed to the presence of higher CO₂ and CO relative abundances in the cometary coma. Aims. We aim to study the effect of CO₂ and CO relative abundances on the observed G/R ratio in comets observed at large (>2 au) heliocentric distances by accounting for important production and loss processes of O(¹S) and O(¹D) atoms in the cometary coma. Methods. Recently we have developed a coupled chemistry-emission model to study photochemistry of O(¹S) and O(¹D) atoms and the production of green and red-doublet emissions in comets Hyakutake and Hale-Bopp. In the present work we applied the model to six comets where green and red-doublet emissions are observed when they are beyond 2 au from the Sun. Results. The collisional quenching of O(¹S) and O(¹D) can alter the G/R ratio more significantly than that due to change in the relative abundances of CO₂ and CO. In a water-dominated cometary coma and with significant (>10%) CO₂ relative abundance, photodissociation of H₂O mainly governs the red-doublet emission, whereas CO₂ controls the green line emission. If a comet has equal composition of CO₂ and H₂O, then ~50% of red-doublet emission intensity is controlled by the photodissociation of CO₂. The role of CO photodissociation is insignificant in producing both green and red-doublet emission lines and consequently in determining the G/R ratio. Involvement of multiple production sources in the O(¹S) formation may be the reason for the observed higher green line width than that of red lines. The G/R ratio values and green and red-doublet line widths calculated by the model are consistent with the observation. Conclusions. Our model calculations suggest that in low gas production rate comets the G/R ratio greater than 0.1 can be used to constrain the upper limit of CO₂ relative abundance provided the slit-projected area on the coma is larger than the collisional zone. If a comet has equal abundances of CO₂ and H₂O, then the red-doublet emission is significantly (~50%) controlled by CO2 photodissociation and thus the G/R ratio is not suitable for estimating CO₂ relative abundance.