C18O observations of the dense cloud cores and star formation in Ophiuchus

In order to reveal the distribution of the dense gas of greater than or equal to 10(4) cm(3), (CO)-O-18 (J = 1-0) observations have been made toward the molecular clouds in the Ophiuchus region of similar to 6.4 deg(2) 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 (CO)-O-18 dense cores are distributed not only in the active star-forming region, rho Oph cloud core, but also in the North region where star formation is less active. The typical core mass, M-LTE, radius, R, and average number density, n(H-2), of the cores are 90 M-circle dot, 0.24 pc, and 1.7 x 10(4) cm(-3) in the rho Oph region, respectively, and 14 M-circle dot, 0.19 pc, and 7.6 x 10(3) cm(-3) in the North region, respectively. Nine of the 40 cores are associated with young stellar objects, and most of the (CO)-O-18 cores are starless. An analysis of the physical parameters of the (CO)-O-18 COTES show that star-forming cores tend to have larger N(H-2) than the rest by a factor of similar to 3, although there is no significant trend in the other physical parameters between star forming and starless cores. We have compared the present (CO)-O-18 data with the (CO)-C-13 data (Nozawa et al.) and with the associated YSOs, in order to understand better the condensing process from molecular gas with density of similar to 10(3) cm-3 to protostars. It is found that 55% of the (CO)-C-13 cores are associated with (CO)-O-18 cores and that the (CO)-O-18 cores are typically less massive, smaller and denser by similar to 34%, similar to 32%, and a factor of similar to 3, respectively, than the (CO)-C-13 cores. It is also found that the (CO)-O-18 cores have steeper density profiles than the (CO)-C-13 cores; when we fit the density profile by a power law as rho proportional to r(-beta), the values of beta for (CO)-O-18 and (CO)-C-13 are estimated as similar to 1.5 and 1.2, respectively. This suggests that the (CO)-O-18 cores are gravitationally more relaxed than the (CO)-C-13 cores. In order to investigate the energetics of the cores, the virial mass, M-VIR, has been calculated for each core. It is found that most of the (CO)-C-13 cores have M-VIR larger than M-LTE. On the other hand, 22 of the 40 (CO)-O-18 cores have M-VIR smaller than M-LTE, suggesting that the (CO)-O-18 cores are more deeply gravitationally bound than the (CO)-C-18 cores. Further, we have found a correlation between the ratio M-VIR/M-LTE and star formation activity: (1) For (CO)-C-13 cores, the fraction of the (CO)-C-13 cores associated with the (CO)-O-18 cores tends to increase with decreasing M-VIR/M-LTE, and (2) for the (CO)-O-18 cores, the fraction of the (CO)-O-18 cores associated with stars tends to increase with decreasing of M-VIR/M-LTE. 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 (CO)-C-13 cores to the (CO)-O-18 cores, they should lose the turbulence energy of similar to 10(44) 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 Alfvenic energy loss may be viable as the dissipation mechanism.