Molecular Clouds: Formation and Disruption

Molecular clouds (MC) are the densest and coldestcomponent of the interstellar gas, and the sites of starformation. They are also turbulent and fractaland theirmasses and sizes span several orders of magnitude. It is also generally believed that they are close to Virial equilibrium (VE).Since this statement has beenquestioned by a number of authors, with important implicationson molecular clouds’ lifetimes, we will review this subjectwithin the context of a turbulent ISM. In this framework, there issignificant numerical evidence that MCs are not in VE, that there is a strong exchange of mass, momentum and energy between clouds and their surrounding medium, andthat it is difficult (if not impossible) to form quasistatic coresinside MCs, suggesting that they must be transient, short-livedphenomena. Thus, their formation and disruption must be primarily dynamical, and probably not due tojust a single mechanism, but rather to the combination of severalprocesses. This picture seems consistent withrecent estimates of ages of stars in the solar neighborhood.