Study on Molecular Cloud Formation by Large scale Submillimeter-wave Observations of Neutral Carbon Atom

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In the present study, we have developed the sensitive receiver system installed on the Mount Fuji submillimeter-wave telescope in order to conduct the large-scale mapping observations of the neutral carbon atom. We achieved the DSB noise temperature of 120 K at 492 GHz and 580 K at 809 GHz. These are the top class performance among the submillimeter-wave receivers continuously used in the astronomical observations.With this sensitive receiver, we have conducted large-scale mapping observation of the 492 GHz line of the neutral carbon atom toward many nearby molecular clouds including Orion A giant molecular cloud and Taurus dark cloud complex. Furthermore, we have mapped several representative star-forming regions with the 809 GHz line of the neutral carbon atom for the first time. From these results, we found that the abundance of the neutral carbon atom is not strongly dependent on the UV field strength, but on the evolutionary stage of molecular clouds.