= Cooling-limited disk fragmentation
A <gas> disk near $Q_T\sim1$ need not form bound fragments: gravitational compression and shocks can heat it into a self-regulated spiral state. Fast enough cooling, often parameterized by $\beta=\Omega t_{\rm cool}$, permits collapse to continue. A numerical critical beta depends on thermodynamics, dimensionality and the cooling model; it is not universal.<Nonlinear Outcome of Gravitational Instability in Cooling, Gaseous Disks by Charles F. Gammie>[Local razor-thin cooling experiment and its model-specific fragmentation threshold]{ref}
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