Observational Studies on the Euolution of Protoplanetary Disks

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Progress was made on understanding the formation process of protoplanetary disks with the aid of this grant. In summary, a protoplanetary disk grows around a protostar in the embedded phase of early stellar evolution through accreting gas from its envelope with -10,000 AU in size. The star appears as a T Tauri star with a surrounding protoplanetary disk after the envelope is dissipated due to accretion. Details are summarized as follows.1. Observations of Taurus molecular cloud with the University of Tokyo-Nobeyama Radio Observatory 60 cm telesope in the CO (J=2-1) line revealed that the molecular cloud as a whole exhibits relatively low value of 0.6 for the ratio of the J=2-1 to J=1-0 intensities, suggesting that the entire molecular cloud is less dense and cold. This may affect to produce only low mass starts in this cloud.2. A C^<18>O (J=1-0) survey was made toward 32 IRAS sources associated with the Taurus molecular cloud. Younger sources as indicated by their redder IRAS colors tend to have larger amount of associated gas, while the bluer, i.e., older, sources do not. This means that the gas in the envelope of protostars decreases in the course of evolution from embedded to visible stars.3. Dynamical accretion was first detected from the gas disk aroud HL Tau, suggesting that HL Tau is in the transitional phase from an embedded protostar to a T Tauri star. A protoplanetary disk of -40 AU is forming around HL Tau.4. CO and its isotopic emissions have been detected from the disks around GG Tau and DM Tau. It was found that heavy molecules are depleted in disks.