Ammonia is one of the major N-bearing molecules in cometary ices. Ammonia can be observed by radio (inversion transitions), far-infrared (rotational transition) and near-infrared regions (vibrational transitions). In the near-infrared regions, not only ammonia but also water and other organic volatiles can be observed as emission lines simultaneously by the high-dispersion spectroscopy. Therefore, it is easy to determine mixing ratios of various organic volatiles from the near-infrared high-dispersion spectra of comets.
The fluorescence efficiencies for the emission lines are necessary to determine the gas production rate from the observations, and there are a few studies for the fluorescence excitation model of ammonia in comets. The population distribution in the vibrational ground state is assumed to follow the Boltzmann distribution in these models. This assumption is valid in the case of inner coma of productive comets (i.e., frequent inter-molecular collisions can maintain the Boltzmann distribution). In the low-activity comets, however, the inter-molecular collisions are not so frequent enough to maintain the Boltzmann distribution in the vibrational ground state. In the case of ammonia, the population will favor the metastable states if the collision time scale is much longer than the decay time scales 10 s for the non-metastable states.
We developed the fluorescence excitation model of cometary ammonia by involving collisional transitions explicitly. Transitions by the collision with water and by the collision with electron are taken into account. We will apply our model to the observations of Jupiter family comets.
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