the evolution from cloud core to protostar is dominated by gravity, the physics controlling the process in which the cores themselves, and the clouds in which they are embedded, are formed and evolve is still quite controversial. While on the scale of pc to tens of pc molecular clouds are close to satisfying virial equilibrium between gravitational and kinetic energies, the significance of this equality is not entirely clear. Furthermore, the role of magnetic field, while often postulated to be significant, remains uncertain [@shu1987; @heiles2005]. Finally, the formation of molecular clouds themselves, and their lifetime, remains very much a matter of discussion [e.g. @hartmann2001]

Molecular clouds may be formed by compression of atomic gas, with the increased density and extinction enhancing the formation rate of molecules, starting with 2, for which self–shielding enables the buildup of a substantial fraction of the total hydrogen density even when the visual extinction  is only a fraction of a magnitude. It has also been suggested that the large molecular cloud presence in galactic spiral arms is the result of the agglomeration of molecular material existing in the interarm region, as discussed by [@pringle2001]. While one viewpoint has held that molecular clouds have relatively long 