= Thin disk
{wiki}
= Thin disc
{synonym}
A \b[<thin disk>] has <disk aspect ratio> $H/r\ll1$, where $H$ is its vertical thickness scale and $r$ its <cylindrical radius>. The separation lets radial orbital motion and vertical <hydrostatic equilibrium> be treated at different orders. In a centrally dominated gas disk, $H/r\sim c_s/v_K$: a low <sound speed> compared with orbital speed naturally produces a flattened layer. Geometrical thinness alone does not imply large <optical depth> or efficient <radiative cooling>.
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