Six myths on the virial theorem for interstellar clouds

Abstract

The interstellar medium is highly dynamic and turbulent. However, little or no attention has been paid in the literature to the implications that this fact has on the validity of at least six common assumptions on the virial theorem (VT), which are as follows. (i) The only role of turbulent motions within a cloud is to provide support against collapse; (ii) the surface terms are negligible compared to the volumetric ones; (iii) the gravitational term is a binding source for the clouds since it can be approximated by the gravitational energy; (iv) the sign of the second time-derivative of the moment of inertia determines whether the cloud is contracting or expanding ⁠; (v) interstellar clouds are in virial equilibrium (VE) and (vi) Larson's relations (mean density–size and velocity dispersion–size) are the observational proof that clouds are in VE. However, turbulent, supersonic interstellar clouds cannot fulfil these assumptions because turbulent fragmentation will induce flux of mass, moment and energy between the clouds and their environment, and will favour local collapse while it may also disrupt the clouds within a dynamical time-scale. It is argued that although the observational and numerical evidence suggests that interstellar clouds are not in VE, the so-called ‘virial mass’ estimations, which should actually be called ‘energy-equipartition mass’ estimations, are good order of magnitude estimations of the actual mass of the clouds just because observational surveys will tend to detect interstellar clouds appearing to be close to energy equipartition. Similarly, order of magnitude estimations of the energy content of the clouds is reasonable. However, since clouds are actually out of VE, as suggested by asymmetrical line profiles, they should be transient entities. These results are compatible with observationally based estimations for rapid star formation, and call into question the models for the star formation efficiency based on clouds being in VE.