Fluorescence Excitation Models of Ammonia and Amidogen Radical (NH2) in Comets: Application to Comet C/2004 Q2 (Machholz)
Ammonia is a major reservoir of nitrogen atoms in cometary materials. However, detections of ammonia in comets are rare, with several achieved at radio wavelengths. A few more detections were obtained through near-infrared observations (around the 3 m wavelength region), but moderate relative velocity shifts are required to separate emission lines of cometary ammonia from telluric absorption lines in the 3 micron wavelength region. On the other hand, the amidogen radical (NH2 -- a photodissociation product of ammonia in the coma) also shows rovibrational emission lines in the 3 micron wavelength region. Thus, gas production rates for ammonia can be determined from the rovibrational emission lines of ammonia (directly) and amidogen radical (indirectly) simultaneously in the near-infrared. In this article, we present new fluorescence excitation models for cometary ammonia and amidogen radical in the near-infrared, and we apply these models to the near-infrared high-dispersion spectra of comet C/2004 Q2 (Machholz) to determine the mixing ratio of ammonia to water in the comet. Based on direct detection of NH3 lines, the mixing ratio of NH3/H2O is 0.46% +/- 0.03% in C/2004 Q2 (Machholz), in agreement with other results. The mixing ratio of ammonia determined from the NH2 observations (0.31% -- 0.79%) is consistent but has relatively larger error, owing to uncertainty in the photodissociation rates of ammonia. At the present level of accuracy, we confirm that NH3 could be the sole parent of NH2 in this comet.