(CO)-O-18 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 equ al to 10(4) cm(3), (CO)-O-18 (J = 1-0) observations have been made toward t he molecular clouds in the Ophiuchus region of similar to 6.4 deg(2) with t he two 4 m telescopes of Nagoya University. Forty dense cores have been ide ntified, providing the first complete sample of such dense cores in Ophiuch us. The (CO)-O-18 dense cores are distributed not only in the active star-f orming 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 aver age 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 o f 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) tha n the rest by a factor of similar to 3, although there is no significant tr end in the other physical parameters between star forming and starless core s. 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 co ndensing process from molecular gas with density of similar to 10(3) cm-3 t o protostars. It is found that 55% of the (CO)-C-13 cores are associated wi th (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 simi lar 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; wh en 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 gr avitationally 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 core s 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 decreasin g 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 int ernal turbulence leads to formation of denser cores and subsequent star for mation. Through the evolution from the (CO)-C-13 cores to the (CO)-O-18 cor es, they should lose the turbulence energy of similar to 10(44) ergs. The s upersonic gas motion with the magnetic fields produces shocks, and the radi ation from the small shocked region may significantly contribute to the coo ling. We suggest that the cores have continuous collisions between turbulen t eddies to produce the C-shocks. Also, the Alfvenic energy loss may be via ble as the dissipation mechanism.