Revisit to the Nuclear Spin Temperature of NH$_{3}$ in Comet C/2001 Q4 (NEAT) Based on High-Dispersion Spectra of Cometary NH$_{2}$

Abstract

Abstract The Ortho-to-Para abundance ratio (OPR) of cometary molecules is considered to be one of primordial characters of comeary ices, which has information concerning their formation. In order to determine the OPR of cometary ammonia (NH$_{3}$), high-dispersion spectra in the optical wavelength region have been the most powerful tool, since NH$_{2}$ is a photo-dissociation product of NH$_{3}$, and NH$_{2}$ has a strong electronic transition in the optical region. Here we present an improved procedure to derive the OPR of NH$_{2}$ (OPR of NH$_{3}$ is derived from that of NH$_{2}$) from the high-dispersion spectra of NH$_{2}$ in comets. We consider (1) the contamination of NH$_{2}$ lines by the C$_{2}$ Swan-band lines, (2) the influence by the telluric absorption lines, and (3) multiple measurements of OPR from different vibronic bands of NH$_{2}$ in the optical region, to achieve a more accurate determination of OPRs than that in previous studies. We applied the new method to the spectrum of comet C/2001 Q4 (NEAT) for which the OPR of NH$_{2}$ was determined in the previous study. The derived OPR of NH$_{2}$ is 3.23$ \pm $0.03, and this is more accurate than before. The OPR of NH$_{3}$ is estimated to be 1.12$ \pm $0.02, which corresponds to a nuclear spin temperature ($T_\mathrm{spin}$) of 30$^{+2}_{-1} $K. The $T_\mathrm{spin}$ value of NH$_{3}$ is basically consistent with the temperature inferred from the mixing ratio of CO with respect to H$_{2}$O found in the same comet.