Photochemistry of atomic oxygen green and red-doublet emissions in comets at larger heliocentric distances
Space Physics Laboratory, Vikram Sarabhai Space Center, 695022 Trivandrum, India
Received: 20 May 2013
Accepted: 28 March 2014
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.
Key words: molecular processes / comets: general
© ESO, 2014