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

Presentation #102.05 in the session “Chemistry in Cometary Comae”.

Published onOct 03, 2021
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.


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