Accretion-disk boundary layer ($T_{\mathrm{BL}},L_{\mathrm{BL}}$)

An accretion-disk boundary layer connects nearly Keplerian rotation to a stellar surface rotating at a different angular speed. For a slowly rotating star, dissipation of the incoming orbital kinetic energy gives $L_{\mathrm{BL}}\simeq GM\dot{M}/(2r_{\ast })$, comparable to the standard thin-disk luminosity. If the emitting area scales as $r_{\ast }H$, its blackbody effective temperature scales as $T_{\mathrm{BL}}\sim (r_{\ast }/H{)}^{1/4}{T}_{\mathrm{in}}$. The coefficient depends on geometry, and stellar rotation reduces the available energy.