ORTHO-TO-PARA ABUNDANCE RATIO OF WATER ION IN COMET C/2001 Q4 (NEAT): IMPLICATION FOR ORTHO-TO-PARA ABUNDANCE RATIO OF WATER*

摘要

The ortho-to-para abundance ratio (OPR) of cometary molecules is considered to be one of the primordial characteristics of cometary ices, and contains information concerning their formation. Water is the most abundant species in cometary ices, and OPRs of water in comets have been determined from infrared spectroscopic observations of H2O rovibrational transitions so far. In this paper, we present a new method to derive OPR of water in comets from the high-dispersion spectrum of the rovibronic emission of H2O+ in the optical wavelength region. The rovibronic emission lines of H2O+ are sometimes contaminated by other molecular emission lines but they are not affected seriously by telluric absorption compared with near-infrared observations. Since H2O+ ions are mainly produced from H2O by photoionization in the coma, the OPR of H2O+ is considered to be equal to that of water based on the nuclear spin conservation through the reaction. We have developed a fluorescence excitation model of H2O+ and applied it to the spectrum of comet C/2001 Q4 (NEAT). The derived OPR of water is 2.54+0.32 – 0.25, which corresponds to a nuclear spin temperature (T spin) of 30+10 – 4?K. This is consistent with the previous value determined in the near-infrared for the same comet (OPR?= 2.6?± 0.3, T spin?= 31+11 – 5?K).