Two models of molecular cloud dynamics are in the literature. On one hand, the “turbulent” one, where MCs are supported by turbulence over several free-fall times. On the other hand, the “hierarchical and chaotic global collapse” one, which states that as MCs form, they collapse, form stars and disperse, all within a few free-fall times. In this one, the non-thermal motions that people interpret as turbulence are generated by the collapse itself, but they are hardly a mechanism that supports clouds.
Both models of MCs reproduce important observational features of MCs. Using Gaia EDR3, we have found what appears to be a distinctive feature that may allow us to disentangle which model of cloud formation and evolution is at play in MCs: the velocity dispersion of the newborn stars as a function of the mass of the stars. While turbulence models show that massive stars rapidly acquire high velocity dispersion, collap-sing models show the distinctive signs of violent relaxation, where the velocity dispersion does not depend on the mass bin of the stars.