Study on formation condition of cometary ice on the basis of ortho-to-para ratio of ammonia

Comets are thought to be relics of our solar system, which have kept the information on solar system formation. The solar system formed from a collapsing molecular cloud. Many planetesimals formed first in the proto- planetary disk around proto-Sun (called "solar nebula"). Typical dimen- sions of the planetesimals were between a few hundreds meters and a few kilometers. They accreted into planets and satellites, while some of them remained as fragments of collisions which are asteroids, and the remnants of icy planetesimals survived in outer solar system as comets or small icy bodies. Therefore, studies on physical conditions of the solar nebula from a viewpoint of comets are important for investigating the formation process or circumstance of the solar system.There are some primordial properties in comets, e.g, a chemical abun- dance, isotopic ratios for various elements, and ortho-to-para ratios (OPRs) of cometary materials. The OPR is an important character of cometary molecules which have hydrogen atoms at symmetrical positions. A spin temperature derived from OPR could reflect the conditions (especially, a temperature) where the molecule formed. The spin temperature is thought to indicate a temperature of grains in the solar nebula because the molecules formed in the icy mantles on grains.Among cometary species there are no reliable reports except water molecules, of which the spin temperature has been derived as about 30 K for several comets. This temperature is consistent with the temperature range inves- tigated from deuterium-to-hydrogen (D/H) ratios of water and hydrogen cyanide, and from abundances of argon and neon in comets. In this thesis, a new method to investigate OPR of ammonia in comets, along with first applications of this method to two Oort cloud comets, comet C/1999S4 (LINEAR) and comet C/2001A2 (LINEAR), is presented. Ammonia is important as a product of nitrogen related chemical reactions in the solar nebula, and as a reservoir of nitrogen atoms in comets. However, there are only a few reports on the detection of cometary ammonia for bright comets by radio observations, and no reports on OPR of cometary ammonia. In this thesis, it is shown that the OPR of cometary ammonia can be also determined from OPR of NH2, which is observable in the optical wavelength region. NH2 is thought to be a photodissociation product of cometary ammonia, which is confirmed by the spatial distribution of NH2 in the coma of comet C/1996B2 (Hyakutake). In order to derive the OPR of NH2 from observed emission lines, the fluorescence excitation model is established. As an application of this model to the high-dispersion optical spectra taken by the high dispersion spectrograph (HDS) and the Subaru telescope, the OPRs of NH2 are derived as 3.32±0.09 in comet C/1999S4 (LINEAR) and as 3.43±0.09 in comet C/2001A2 (LINEAR), respectively. These values indicate OPRs of ammonia to be 1.16±0.05 and 1.22±0.05 for comet C/1999S4 (LINEAR) and comet C/2001A2 (LINEAR), respectively. Derived spin temperatures of ammonia, 28+3-2 K for comet C/1999S4 (LINEAR) and 26+2-1 K for comet C/2001A2 (LINEAR), are consistent with temperature ranges investigated for the Oort cloud comets in the previous studies. These results are indicative of grain processing for the formation of cometary ammonia. The derived spin temperatures may indicate the formation region between the orbit of Saturn to that of Uranus in the solar nebula. These comets are thought to originate from Oort cloud concerning their orbits. Based on statistical studies on orbital evolutions of comets in the solar nebula, it is thought that Oort cloud comets formed between the orbits of Jupiter and Neptune and then they were scattered by giant planets into outer edge of the solar system. The result obtained in this study is consistent with the statistical studies on orbital evolution of the Oort cloud comets.Further observations of OPR of cometary ammonia are required, espe- cially, a comparison between Oort cloud comets and Kuiper belt comets (which are thought to form further than the orbit of Neptune in the solar nebula) is important for investigating origin of comets