Statistical study of C18O dense cloud cores and star formation

Dense molecular cloud cores are studied statistically in nearby (d less than or equal to 200 pc) star-forming regions (SFRs) that show various modes of star formation. As a result of the (CO)-O-18 survey of NANTEN and the 4 m radio telescopes of Nagoya University, 179 cores have been collected in the SFRs of Taurus, the rho Oph cloud, the Ophiuchus north region, the Lupus clouds, L1333, the Corona Australis cloud, Southern Coalsack, and the Pipe nebula, and their physical properties investigated. According to their star-formation activities, the cores are divided into 3 categories as 136 starless, 36 star-forming, and 7 cluster-forming cores. It is found that cores with active star formation tend to have larger N(H-2), n(H-2), and M. The mass function of the cores does not appear to follow a single power-law function, but the power-law index is subject to change with the mass range. The average star-formation efficiency (SFE) of the cores is roughly similar to10%, and the expected stellar mass function from the SFE approximates the stellar initial-mass function (IMF). Virial analysis shows that the star-forming cores are gravitationally more bound, with smaller virial ratios than the starless cores, while cluster-forming cores are marginally bound with moderate virial ratios. We found that turbulent decay is indicated by diminishing DeltaV from the starless to the star-forming cores. It is suggested that the turbulent decay is necessary for star formation, while formed star clusters provide the turbulence and make the cores unbound. Molecular clouds associated with the clusters tend to have head-tail structures and the cluster formation takes place at the head. This implies that the clouds are affected by external shocks, which have triggered cluster formation. We suggest that star and cluster formation are strongly controlled by the initial amount of internal turbulence and the interaction with the external shocks.