The Virial Balance of Molecular Clumps and Cores in Colliding Magnetized Flows

Abstract

We simulate the formation of molecular clouds in colliding flows of warm neutral medium with the adaptive mesh refinement code {\sc Flash}. We include a chemical network to treat heating and cooling and to follow the formation of molecular gas. For the forming molecular clumps and cores in four different simulations with varying initial magnetic field strength between 0.01 - 5\(\,\mu\)G, we carry out a full virial analysis including all time-independent surface and volume terms as well as the time-dependent term. The initial magnetic field strength influences the fragmentation properties of the forming cloud because it prohibits motions perpendicular to the field direction and hence alters, or even suppresses, the formation of filamentary substructures. Molecular clump and core formation occurs anyhow. As a result, with increasing field strength, we find more fragments with a smaller average mass; yet the initial field strength is dynamically not relevant for the fragments which constitute our molecular clumps and cores. %yet the magnetic field overall is dynamically negligible for the fragments which constitute our molecular clumps and cores. The molecular clumps are mostly unbound, probably transient objects, which seem to be weakly confined by ram pressure or thermal pressure, indicating that they are swept up by the turbulent flow. They experience significant fluctuations in the mass flux through their surface, indicating that the Eulerian reference frame gives rise to a dominant time-dependent term due to their ill-defined nature. We define the cores to encompass molecular gas, which is additionally highly shielded. Most cores are in gravitational-kinetic equipartition and are already well described by the common virial parameter \(\alpha_\mathrm{vir}\) (as can be seen from the Heyer relation), while some undergo minor dispersion by kinetic surface effects.