Revisit to the nuclear spin temperature of NH3 in comet C/2001 Q4 (NEAT) based on high-dispersion spectra of cometary NH2

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 (NH3), high-dispersion spectra in the optical wavelength region have been the most powerful tool, since NH2 is a photo-dissociation product of NH3, and NH2 has a strong electronic transition in the optical region. Here we present an improved procedure to derive the OPR of NH2 (OPR of NH3 is derived from that of NH2) from the high-dispersion spectra of NH2 in comets. We consider (1) the contamination of NH2 lines by the C2 Swan-band lines, (2) the influence by the telluric absorption lines, and (3) multiple measurements of OPR from different vibronic bands of NH2 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 NH2 was determined in the previous study. The derived OPR of NH2 is 3.23 ± 0.03, and this is more accurate than before. The OPR of NH3 is estimated to be 1.12 ±0.02, which corresponds to a nuclear spin temperature (Tspin) of 30 +2-1 K. The Tspin value of NH3 is basically consistent with the temperature inferred from the mixing ratio of CO with respect to H2O found in the same comet. © 2010. Astronomical Society of Japan.