Fixed Surface Pressure

When the atm_option control has the value 'fixed_Psurf', MESA sets \(P_{\rm surf}\) to the value specified by the atm_fixed_Psurf control.

Then, MESA sets \(T_{\rm surf}\) from the Eddington-grey \(T(\tau)\) relation

\[T_{\rm surf}^{4} = \frac{3}{4} T_{\rm eff}^{4} \left( \tau_{\rm surf} + 2/3 \right).\]

The effective temperature \(T_{\rm eff}\) is determined from the stellar radius and luminosity; see here for a discussion of how the surface optical depth \(\tau_{\rm surf}\) is evaluated.