Direct Simulation Monte-Carlo Model of Forbidden Oxygen Emission Lines in Comets

The main components of cometary ices (H₂O, CO₂ and CO) are precious clues leading to the solar system formation. Although both H₂O and CO can be observed from the ground-based facilities through the infrared wavelength window, CO₂ cannot be accessed from the ground due to severe atmospheric absorption. Consequently, cometary CO₂ can only be observed with space-telescopes or spacecrafts.

When metastable oxygen atom (¹S) produced by the photo-dissociation of H₂O (or other O-bearing molecules) and decays to the excited state (¹D) without collisions, the forbidden emission lines at 557.7nm (“green line”) can be observed. The oxygen atom in ¹D state emits the “red doublet” (at 630.0nm, 636.4nm) associated with the radiative transition to the ground state. These forbidden oxygen emission lines are usually observed in cometary spectrum as a proxy of the CO₂/H₂O mixing ratio in comets. A model of forbidden oxygen emission in cometary coma is necessary to interpret observed forbidden oxygen emission lines and to determine the CO₂/H₂O ratio. However, present models are insufficient consideration of detailed dynamics of products (molecules or atoms) in photo-dissociation reactions.

We construct a model of forbidden oxygen emission in comets by taking account internal energies of photo-dissociation products (e.g. rovibrational energies of H₂ in the case of photo-dissociation of H₂O to H₂ and O) based on the Direct Simulation Monte-Carlo (DSMC) technique. We present our model and application results to the observations of forbidden oxygen emission in comets reported so far.