C18O Observations of the Dense Cloud Cores and Star Formation in Ophiuchus

In order to reveal the distribution of the dense gas of ≥104 cm3, C18O (J = 1-0) observations have been made toward the molecular clouds in the Ophiuchus region of ~6.4 deg2 with the two 4 m telescopes of Nagoya University. Forty dense cores have been identified, providing the first complete sample of such dense cores in Ophiuchus. The C18O dense cores are distributed not only in the active star-forming region, ρ Oph cloud core, but also in the North region where star formation is less active. The typical core mass, MLTE, radius, R, and average number density, n(H2), of the cores are 90 M☉, 0.24 pc, and 1.7 × 104 cm-3 in the ρ Oph region, respectively, and 14 M☉, 0.19 pc, and 7.6 × 103 cm-3 in the North region, respectively. Nine of the 40 cores are associated with young stellar objects, and most of the C18O cores are starless. An analysis of the physical parameters of the C18O cores show that star-forming cores tend to have larger N(H2) than the rest by a factor of ~3, although there is no significant trend in the other physical parameters between star forming and starless cores. We have compared the present C18O data with the 13CO data (Nozawa et al.) and with the associated YSOs, in order to understand better the condensing process from molecular gas with density of ~103 cm-3 to protostars. It is found that 55% of the 13CO cores are associated with C18O cores and that the C18O cores are typically less massive, smaller and denser by ~34%, ~32%, and a factor of ~3, respectively, than the 13CO cores. It is also found that the C18O cores have steeper density profiles than the 13CO cores; when we fit the density profile by a power law as ρ ∝ r-β, the values of β for C18O and 13CO are estimated as ~1.5 and 1.2, respectively. This suggests that the C18O cores are gravitationally more relaxed than the 13CO cores. In order to investigate the energetics of the cores, the virial mass, MVIR, has been calculated for each core. It is found that most of the 13CO cores have MVIR larger than MLTE. On the other hand, 22 of the 40 C18O cores have MVIR smaller than MLTE, suggesting that the C18O cores are more deeply gravitationally bound than the 13CO cores. Further, we have found a correlation between the ratio MVIR/MLTE and star formation activity: (1) For 13CO cores, the fraction of the 13CO cores associated with the C18O cores tends to increase with decreasing MVIR/MLTE, and (2) for the C18O cores, the fraction of the C18O cores associated with stars tends to increase with decreasing of MVIR/MLTE. We interpret this to indicate that the gradual dissipation of the internal turbulence leads to formation of denser cores and subsequent star formation. Through the evolution from the 13CO cores to the C18O cores, they should lose the turbulence energy of ~1044 ergs. The supersonic gas motion with the magnetic fields produces shocks, and the radiation from the small shocked region may significantly contribute to the cooling. We suggest that the cores have continuous collisions between turbulent eddies to produce the C-shocks. Also, the Alfvénic energy loss may be viable as the dissipation mechanism.