Abstract
We carried out high-dispersion, spectroscopic observations of comet C/2001 Q4 (NEAT) in the near-infrared with the 8 m Subaru telescope and detected the R-branch emission series of the ν3 vibrational band of methane. The signal-to-noise ratio of the observed spectrum was sufficient to make the first determination of the nuclear spin temperature of methane, derived to be 33 K, which reflects the temperature of formation or condensation of molecules on cold grains. The upper limit of the CH3D/CH4 ratio was determined to be 0.04 (95% confidence limit), indicating the formation of methane in a dense molecular cloud at temperatures higher than about 30 K. On the basis of these observational results, we conclude that the Sun was born in a warm molecular cloud near 30 K, not in a cold dark cloud near 10 K, as is usually assumed.
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