2009ApJ...703..121K

We observed comet C/2004 Q2 (Machholz) with the Keck II telescope in late 2005 January and we obtained the spectra of C/2004 Q2 including many emission lines of volatile species such as H₂O, HCN, C₂H₂, NH₃, CH₄, C₂H₆, CH₃OH, and H₂ CO with high-signal-to-noise ratios. Based on our observations, we determined the mixing ratios of the molecules relative to H₂ O in C/2004 Q2. Since C/2004 Q2 is one of Oort Cloud comets, it is interesting to compare our results with other Oort Cloud comets. The mixing ratios of C₂H₂/H₂ O and C₂H₆/H₂O in C/2004 Q2 are lower than typical Oort Cloud comets. Especially, C₂H₂/H₂ O ratio in C/2004 Q2 is as lower as Jupiter Family comets. However, mixing ratios of other molecules in C/2004 Q2 are similar to typical Oort Cloud comets. C/2004 Q2 might be the intermediate type between Oort Cloud and Jupiter Family comets. To investigate the formation conditions of such intermediate type comet, we focused on the (C₂H₂+C₂H₆)/H₂O ratios and C₂H₆/(C₂H₆+C₂H₂) ratios in comets from the viewpoint of conversion from C₂H₂ to C₂H₆in the precometary ices. We found that (C₂H₂+C₂H₆)/H₂ O ratio in C/2004 Q2 is lower than the ratio in typical Oort Cloud comets while C₂H₆/(C₂H₆+C₂H₂) ratio in C/2004 Q2 is consistent with the ratio of the typical Oort Cloud comets and Jupiter family comets. If we assume that the cometary volatiles such as H₂O, CH₄, and C₂H₂ formed similar environment, the C₂H₆/(C₂H₆+C₂H₂) ratio might not be sensitive in the temperature range where hydrogen-addition reactions occurred and cometesimals formed (∼30 K). We employed the dynamical-evolutional model and the chemical-evolutional model to determine the formation region of C/2004 Q2 more precisely. We found that comet C/2004 Q2 might have formed in relatively inner region of the solar nebula than the typical Oort Cloud comet (but slightly further than 5 AU from the proto-Sun).