2009ApJ...693..388K

We report the nuclear spin temperatures (T_spin) of H₂ O and CH₄ in comet C/2004 Q2 (Machholz) based on high-dispersion spectroscopic observations of the comet in the L band taken by the Keck II telescope with the NIRSPEC spectrograph on 2005 January 30.2 UT. The ortho-to-para abundance ratio of H₂ O was determined to be 3.13^+0.56_–0.42, which is consistent with the high-temperature limit of H₂ O (3.0). The lower limit of T_spinis 27 K (95% confidence limit (c.l.)) for H₂O. In the case of CH₄, the abundance ratios of A:E:F are consistent with 5:2:9 (high-temperature limit) and the lower limit of T_spin is 36 K (95%-c.l.). We also report the result of our sensitive search for mono-deuterated methane (CH₃D) in C/2004 Q2 (Machholz). A candidate for the emission line from the CH₃ D ν₄^RR(3, 3) transition was found in our spectra. Although the D/H ratio in methane could be determined as (3.8±1.3)x10^–3 based on a tentative detection of CH₃D, this emission line also might be assigned to other molecules, e.g., CH₃OH. Therefore, we conclude that the D/H ratio in methane is lower than 6.4x10^–3 (95%-c.l.) in the comet. Both the lower limits of T_spin of H₂O and CH₄and the upper limit of the D/H ratio in methane indicate the formation of molecules frozen in C/2004 Q2 (Machholz) at relatively higher temperatures compared with the typical Oort Cloud comets.