Formation of Massive Protostellar Clusters -- Observations of Massive 70 $μ$m Dark Molecular Clouds

Abstract: We present Submillimeter Array (SMA) observations of seven massive molecular clumps which are dark in the far-infrared for wavelengths up to 70 $\mu$m. Our 1.3 mm continuum images reveal 44 dense cores, with gas masses ranging from 1.4 to 77.1 M${\odot}$. Twenty-nine dense cores have masses greater than 8 M${\odot}$ and the other fifteen dense cores have masses between 1.4 and 7.5 M$_{\odot}$. Assuming the core density follows a power-law in radius $\rho \propto r^{-b}$, the index $b$ is found to be between 0.6 and 2.1 with a mean value of 1.3. The virial analysis reveals that the dense cores are not in virial equilibrium. CO outflow emission was detected toward 6 out of 7 molecular clumps and associated with 17 dense cores. For five of these cores, CO emissions appear to have line-wings at velocities of greater than 30 km s$^{-1}$ with respect to the source systemic velocity, which indicates that most of the clumps harbor protostars and thus are not quiescent in star formation. The estimated outflow timescale increase with core mass, which likely indicates that massive cores have longer accretion timescale than that of the less massive ones. The fragmentation analysis shows that the mass of low-mass and massive cores are roughly consistent with thermal and turbulent Jeans masses, respectively.