Origin of Infrared Point Sources Isolated from Molecular Clouds

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The purpose of this project is to investigate the origin of protostellar candidates isolated from molecular clouds. These sources were first detected by IRAS satellite in 1983 as infrared point sources. They are accompanied by the cold color spectrum at the wave lengths 25-60 micron, which is a characteristic feature of protostars forming in molecular clouds. However, on the way of a large-scale ^<13>CO (J=1-0) survey for molecular clouds in the Cygnus region (80<l[deg]<100) that we made earlier using the Nagoya 4m radio telescopes (Dobashi et al.1994, Astrophysical Journal Suppl., vol.94, p.419), we found that as much as >60% of these cold point sources are located outside dense gaseous material traced in ^<13>CO.These protostellar candidates away from molecular clouds were our particular interest, and we attempted to investigate their origin.On an early stage of the research, we expected that these sources might be really protostars whose parent clouds escaped detection in the earlie … More r ^<13>CO observations for some reasons. It was also expected that a part of these sources should be galaxies or planetary nebulae whose have infrared properties are similar to protostars. In order to examine all these possibilities, we achieved sensitive ^<12>CO observations using the Nobeyama 45m telescope having the high angular resolution 17"toward -250 isolated protostar candidates isolated from molecular clouds. As the result, we detected ^<12>CO emission toward -70% of the isolated sources, indicating that they are likely to be protostars associated with the molecular clouds. About a half of these clouds are highly compact molecular clouds with a diameter of a few arcminuites, and the rest show very high radial velocities indicating that they are quite distant clouds, for which they escaped detection in the earlier ^<13>CO observations. The rest of the sources (-30%) remains unknown, and we are still investigating their origin. Finally, we have extended the search for such isolated protostelar candidates over a greater portion of the galactic plane (80<l[deg]<180) in order to check their ubiquitousness. The result implies that such isolated sources are rich in the sky, occupying 50-60% of all the infrared sources with protostar like infrared properties. Less