Permalink : http://hdl.handle.net/2115/34717

彗星は，46億年前の太陽系形成初期に形成された氷小天体の残存物であり，太陽による加熱等の影響をほとんど受けていない始原天体である．本稿では，筆者らの最近の研究成果を中心にして，彗星核の均質性，起源，形成環境などについて述べる．特に，多くの破片に分裂したSchwassmann-Wachman 3彗星の観測結果から分裂前の彗星核の均質性について議論すると共に，Machholz彗星の観測結果から得られた分子の原子核スピン温度や重水素/水素比を手がかりとして彗星物質の起源について議論する．
Recently we performed near-infrared high dispersion spectroscopic observations of two bright comets, C/2004 Q2 (Machholz) and a nucleus "B" of 73P/Schwassmann-Wachmann 3(SW3). The former was very bright and it allowed us to obtain very high-S/N ratio spectra. We extracted H2O and CH4 emission lines and determined nuclear spin temperatures of both molecules. The obtained temperatures are higher than the typical value for the Oort Cloud comets (～30K). Furthermore, we report a tentative detection of CH3D in comet Machholz. The D/H ratio in methane was consistent with the temperature range obtained from nuclear spin temperatures. On the other hand, in the case of break-up comet 73P/SW3, we also derived higher nuclear spin temperature than the typical Oort Cloud comets. We also found the similarity in chemical composition between fragments of the same comet. Fragments "B" and "C" of comet SW3 are very similar to each other from the viewpoint of chemistry. This fact indicates that the parent body of SW3 was quite homogeneous in chemical composition of ices.