2000ApJ...528..817T

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, M_LTE, radius, R, and average number density, n(H₂), of the cores are 90 M_☉, 0.24 pc, and 1.7x10⁴ cm^–3 in the ρ Oph region, respectively, and 14 M_☉, 0.19 pc, and 7.6x10³ 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, M_VIR, has been calculated for each core. It is found that most of the ¹³CO cores have M_VIR larger than M_LTE. On the other hand, 22 of the 40 C¹⁸O cores have M_VIR smaller than M_LTE, suggesting that the C¹⁸O cores are more deeply gravitationally bound than the ¹³CO cores.

Further, we have found a correlation between the ratio M_VIR/M_LTE and star formation activity: (1) For ¹³CO cores, the fraction of the ¹³CO cores associated with the C¹⁸O cores tends to increase with decreasing M_VIR/M_LTE, and (2) for the C¹⁸O cores, the fraction of the C¹⁸O 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 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.