Observational approach to molecular cloud evolution Original Research Article

We have constructed a 1.2-m submillimeter-wave telescope at the summit of Mt. Fuji to observe molecular clouds in two CI lines, 3P1–3P0 (492 GHz) and 3P2–3P1 (809 GHz). The telescope has been operated successfully for four observing seasons since the installation on 1998. We have obtained large-scale CI 492 GHz distributions of many molecular clouds, including Orion MC, Taurus MC, DR15, ρ-Oph, DR21, NGC2264, M17, W3, W44, W51, Rosette MC, covering more than 40 square degrees of the sky. The distribution of CI 492 GHz emission is found to be different from those of the 13CO or C18O emission in some clouds. We found C0-rich areas (C/CO ∼ 1) in several dark clouds without strong UV sources. Away from UV sources the spatial sequence appears to be C+/CO/C0. This seems to be inconsistent with the standard photodissociation region picture. These results are discussed in relation to formation processes of molecular clouds and dense cloud cores.