The Virial Balance of Clumps and Cores in Molecular Clouds

We study the instantaneous virial balance of CCs in numerical models of MCs. The models represent a range of magnetic field strengths in MCs from subcritical to nonmagnetic regimes. We identify CCs at different density thresholds and calculate, for each object, the terms that enter the EVT. A CC is gravitationally bound when the gravitational term in the EVT is larger than the amount for the system to be virialized, which is more stringent than the condition that it be large enough to make the total volume energy negative. We also calculate other quantities used to indicate the state of gravitational boundedness: Jeans number Jc, mass-to-magnetic flux ratio μc, and virial parameter αvir. Our results suggest the following: (1) CCs are dynamical out-of-equilibrium structures. (2) The surface energies are of the same order as their volume counterparts. (3) CCs are either in the process of being compressed or dispersed by the velocity field. Yet, not all CCs that have a compressive net kinetic energy are gravitationally bound. (4) There is no one-to-one correspondence between the states of gravitational boundedness as described by the virial analysis or by the other indicators. In general, in the virial analysis, only the inner regions of the objects are gravitationally bound, whereas Jc, αvir, and μc estimates tend to show that they are more bound at the lowest threshold levels and more magnetically supercritical. (5) We observe, in the nonmagnetic simulation, the existence of a bound core with structural and dynamical properties that resemble those of Barnard 68. This suggests that such cores can form in a larger MC and then be confined by the warm gas of a newly formed, nearby H II region.