= Photoevaporative gravitational radius
{title2=$r_g=GM/c_s^2$}
The thermal binding scale defined by equating heated gas <sound speed> to circular <Keplerian orbit> speed. It is $r_g=GM/c_s^2$. The circular-orbit binding energy is $GM/(2r)$ per unit mass, while the gas <specific enthalpy> is of order $c_s^2$. A heated disk surface at radii comparable to or larger than $r_g$ can therefore drive <photoevaporation>. The actual launching radius can differ by a factor of order unity; $c_s$ is not the ballistic escape speed.
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