The Spin Temperature of NH3 in Comet C/1999S4 (LINEAR)

A high-dispersion spectrum of Comet C/1999S4 (LINEAR) was obtained in the optical region with the high-dispersion spectrograph on the Subaru telescope when the comet was 0.863 astronomical units from the Sun before its disin-tegration. We obtained high signal-to-noise ratio emission lines of the cometary NH2 bands from which an ortho-to-para ratio (OPR) of 3.33 6 0.07 was derived on the basis of a ßuorescence excitation model. Assuming that cometary NH2 mainly originates from ammonia through photodissociation, the derived OPR of NH2 molecules should reßect that of ammonia, which provides information on the environment of molecular formation or condensation and of the thermal history of cometary ices. Assuming that the OPR of ammonia in comets was unchanged in the nucleus, the derived spin temperature of ammonia (28 6 2 kelvin) suggests that a formation region of the cometary ammonia ice was between the orbit of Saturn and that of Uranus in the solar nebula. Comets are thought to be relics preserving information from the early solar nebula. Re