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

Abstract

We report the nuclear spin temperatures (T spin) of H 2O 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 H2O was determined to be 3.13 +0.56-0.42, which is consistent with the high-temperature limit of H2O (3.0). The lower limit of T spin is 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 (CH 3D) in C/2004 Q2 (Machholz). A candidate for the emission line from the CH3D ν4RR(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 3D, 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 6.4 × 10-3 (95%-c.l.) in the comet. Both the lower limits of T spin of H2O and CH4 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. © 2009. The American Astronomical Society. All rights reserved.