Complex Organic Molecules in Taurus Molecular Cloud-1

We have observed the millimeter-wave rotational spectral lines of CH₃CHO, H₂CCO, cyclopropenone, and H₂CO toward the cyanoployyne peak of Taurus Molecular Cloud-1 (TMC-1 CP). The spectral line profile of CH₃CHO is found to reveal a well-separated double peak. It is similar to the line profile of CH₃OH, but is much different from those of carbon-chain molecules and C³⁴S. The different line profiles mean different distributions along the line of sight. The similarity of the spectral line profiles between CH₃CHO and CH₃OH suggests that CH₃CHO is mainly formed on dust grains as CH₃OH or through gas-phase reactions starting from CH₃OH. On the other hand, the spectral line profiles of H₂CCO and cyclopropenone are rather similar to those of carbon-chain molecules and C³⁴S, implying their gas-phase productions. H₂CO shows a composite spectral line profile reflecting the contributions of both gas-phase and grain-surface productions. In addition, we have detected the spectral lines of CH₃CHO and HCOOCH₃ toward the methanol peak near TMC-1 CP. We have also tentatively detected one line of (CH₃)₂O. Considering the chemical youth of TMC-1, the present results indicate that fairly complex organic species have already been formed in the early evolutionary phase of starless cores. TMC-1 is thus recognized as a novel source where formation processes of complex organic molecules can be studied on the basis of the line profiles.