The turbulent support and global hierarchical collapse models for molecular clouds compared: differences, convergence, and myths

ABSTRACT We provide a detailed comparison between the ‘turbulent support’ (TS) and ‘global hierarchical collapse’ (GHC) theoretical models for molecular clouds and star formation, as a basis for the interpretation of observations and simulations. We discuss their respective interpretations of the observational data, their differences and similarities, and suggested tests to discern between them. Also, we briefly discuss the evolution of both models towards convergence. TS assumes that star-forming molecular clouds and their substructures are either in approximate virial equilibrium between gravity and turbulence or overvirial, so that the cloud is either confined or expanding, and that its substructures are produced by turbulent compressions. In this scheme, the star formation rate (SFR) is time-independent and determined by the turbulent and gravitational parameters of the clouds, in particular the virial parameter $\alpha _{\rm vir}$. Conversely, GHC is an intrinsically evolutionary model, which assumes that most of the mass of star-forming molecular clouds and their substructures is part of a continuous gravitationally driven flow, each accreting from their parent structure. In it, the clouds’ SFR is time-dependent, increasing until massive stars that can disrupt the clouds eventually appear. GHC interprets non-thermal motions as a mixture of infall and turbulent components, with the relative importance of the former increasing as the objects become evolve. Thus, $\alpha _{\rm vir}$ is an evolving variable of the clouds. Tests that may provide clues to distinguishing between TS and GHC must take into account that the innermost parts of globally gravitationally bound structures may not locally appear bound, and thus the binding may have to be searched for at the largest scale of the structure.