Cooling-limited disk fragmentation

A gas disk near $Q_T\sim 1$ need not form bound fragments: gravitational compression and shocks can heat it into a self-regulated spiral state. Fast enough cooling, often parameterized by $\beta =\Omega t_{\mathrm{cool}}$, permits collapse to continue. A numerical critical beta depends on thermodynamics, dimensionality and the cooling model; it is not universal.^[ref]