Formation Conditions of Icy Materials in Comet C/2004 Q2 (Machholz). Ii. Diagnostics Using Nuclear Spin Temperatures and Deuterium-to-Hydrogen Ratios in Cometary Molecules

We report the nuclear spin temperatures ( T spin ) of H 2 O and CH 4 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 H 2 O was determined to be 3.13 +0.56 −0.42 , which is consistent with the high-temperature limit of H 2 O (3.0). The lower limit of T spin is 27 K (95% confidence limit (c.l.)) for H 2 O. In the case of CH 4 , 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 (CH 3 D) in C/2004 Q2 (Machholz). A candidate for the emission line from the CH 3 D ν 4 R R(3, 3) transition was found in our spectra. Although the D/H ratio in methane could be determined as (3.8 ± 1.3) × 10 −3 based on a tentative detection of CH 3 D, this emission line also might be assigned to other molecules, e.g., CH 3 OH. Therefore, we conclude that the D/H ratio in methane is lower than 6.4 × 10 −3 (95%-c.l.) in the comet. Both the lower limits of T spin of H 2 O and CH 4 and 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.