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

Susarla Raghuram; Anil Bhardwaj

doi:10.1051/0004-6361/201321921

paper

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

arXiv:1405.0375 · doi:10.1051/0004-6361/201321921

Abstract

In comets the atomic oxygen green to red-doublet emission intensity ratio (G/R ratio) of 0.1 has been used to confirm H $_{2}$ O as the parent species producing oxygen emission lines. The larger ( $>$ 0.1) value of G/R ratio observed in a few comets is ascribed to the presence of higher CO $_{2}$ and CO relative abundances in the cometary coma. We aim to study the effect of CO $_{2}$ 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( $^{1}$ S) and O( $^{1}$ D) in the cometary coma. Recently we have developed a coupled chemistry-emission model to study photochemistry of O( $^{1}$ S) and O( $^{1}$ D) 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. In a water-dominated cometary coma and with significant ( $>$ 10%) CO $_{2}$ relative abundance, photodissociation of H $_{2}$ O mainly governs the red-doublet emission, whereas CO $_{2}$ controls the green line emission. If a comet has equal composition of CO $_{2}$ and H $_{2}$ O, then ~50% of red-doublet emission intensity is controlled by the photodissociation of CO $_{2}$ . The G/R ratio values and green and red-doublet line widths calculated by the model are consistent with the observation. 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 CO $_{2}$ relative abundance provided the slit-projected area on the coma is larger than the collisional zone. If a comet has equal abundances of CO $_{2}$ and H $_{2}$ O, then the red-doublet emission is significantly ( $\sim$ 50%) controlled by CO $_{2}$ photodissociation and thus the G/R ratio is not suitable for estimating CO $_{2}$ relative abundance.

12 pages, 9 figures, and 3 tables

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

Abstract

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