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 disintegration. 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 ∓ 0.07 was derived on the basis of a fluorescence excitation model. Assuming that cometary NH₂ mainly originates from ammonia through photodissociation, the derived OPR of NH₂ molecules should reflect 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 ∓ 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.