The Virial Balance of Clumps and Cores in Molecular Clouds

(Abridged) We study the virial balance of clumps and cores (CCs) in a set of 3D simulations of driven, MHD, isothermal molecular clouds (MCs). The simulations represent a range of magnetic field strengths in MCs from subcritical to non-magnetic regimes. We identify CCs at different threshold levels. For each object, we calculate the terms that enter the virial theorem in its Eulerian form as well as quantities commonly used in observational and theoretical work to indicate the state of gravitational binding: the Jeans number J_c, the mass-to magnetic flux ratio mu_c, the virial parameter alpha_c. Our results suggest that a) CCs are dynamical out-of-equilibrium structures.b) The surface energy terms are of the same order than their respective volume terms c) CCs can be either in the process of being compressed by the velocity field and have tau_k>0 or dispersed tau_k<0 (tau_k=surface kinetic energy term). Yet, not all clumps that have tau_k>0 are gravitationally bound.d) There is no 1-to-1 correspondence between the state of the gravitational binding as described by the energy balance analysis (EBA) or as implied by the classical indicators. In general, from the EBA we observe that only the inner regions of the CCs are gravitationally bound, whereas J_c and alpha_vir estimates tend to show that they are more gravitationally bound at the lowest threshold levels. Also, the alpha_vir values indicate a larger number of gravitationally bound objects than what is yielded by the EBA.g) We observe, in the non-magnetic run, the existence of a bound core with all the properties of the Barnard 68 globule (B68). This suggests that B68 like cores can form in a larger MC and then be confined by the warm gas of a newly formed HII region heats the gas around the core and confines it.