Subcritical Cloud vs Supercritical Cloud

We have seen there is a critical mass above which the cloud has no (magneto)hydrostatic configuration but below which the cloud has at least an equilibrium state. This gives us an idea that there are two kind of clouds/cloud cores: that with a mass larger than the critical mass which has to collapse dynamically and that with a mass smaller than the critical mass which is in an equilibrium state. In the density range of $10^4{\rm cm^{-3}}\mbox{\raisebox{0.3ex}{$<$}\hspace{-1.1em} \raisebox{-0.7ex}{... ...box{0.3ex}{$<$}\hspace{-1.1em} \raisebox{-0.7ex}{$\sim$}} 10^{10}{\rm cm^{-3}}$, the interstellar gas is essentially isothermal. In this region a major cooling agent is dusts; that is, the dust is heated by the collision of molecules. The excess energy liberated at the collision increases the dust temperature. Finally the thermal emission from the dust cools down the dust again. By this process, the thermal energy of the gas is reduced. Therefore, we consider the cloud/cloud core is isothermal and study the collapse of the isothermal cloud.

Since the supercritical cloud has no hydrostatic configuration, it must evolve in a dynamical way. On the other hand, since the subcritical cloud is in a static state, it evolves in much longer time-scale of the free-fall time. Such cloud evolves by the effect called the ambipolar diffusion.