Revised fluorescence efficiencies of cometary NH2:: Ammonia abundance in comets

We revisit the excitation mechanism of NH2 molecules in cometary comae. A fluorescence efficiency is one of the key parameters for estimating the amount of molecules from the observed flux of NH2 emission in the optical wavelength region. Including rovibronic transitions, which so far have never been considered, we revised the fluorescence efficiencies of cometary NH2 based on the fluorescence equilibrium model. Using the revised fluorescence efficiencies, we derived the abundance of the ammonia from the estimated NH2 abundance because the NH2 is a photodissociation product of cometary ammonia. This revision not only helped us solve the so-called flux ratio anomaly in the NH2 emission bands that were reported so far, but it also helped us to clear up the inconsistency between the ammonia abundances derived in situ by a spacecraft and those estimated from the ground-based observation of the NH2. The NH2 abundances obtained by spectroscopic observations in the past are reevaluated based on our revised parameters and are revealed to be almost constant, with heliocentric distances from 0.5 through 3 AU. The average abundance in comets is found to be about 0.5% relative to the water, except for comet 21P/Giacobini-Zinner.