an ionizing source. The program allows the user to set the gas density at the illuminated face and to choose from several prescriptions to specify how it varies with distance.

One of these prescriptions is [[constant pressure]{}]{}, which may refer to the gas or the total pressure $$P(x) = P_{\rm gas} + P_{\rm ram} + P_{\rm turb} + P_{\rm mag} + P_{\rm lines} + \Delta P_{\rm rad}$$ where $P_{\rm gas}=nkT$ is the thermal pressure, $P_{\rm ram}=\rho v_{\rm wind}^2$ and $P_{\rm turb}=\rho v_{\rm turb}^2/2$ arise from uniform and turbulent motion, respectively, $P_{\rm mag}=\frac{B^2}{8\pi}$ is the magnetic pressure, $P_{\rm lines}$ is the radiation pressure of trapped emission lines [@FerlandElitzur84; @ElitzurFerland86] and $\Delta P_{\rm rad} \equiv \int \rho\,a_{\rm rad}\,{{\rm d}}x$ compensates for the acceleration $a_{\rm rad}$ due to the absorption of the incident radiation. This term is analogous to $\Delta P_{\rm grav}$ and, to some extent, one could argue that the command should already be named “hydrostatic” rather than “constant pressure” even if it did not account for the effect of gravity. Since $a_{\rm rad}$ is positive (the momentum of the absorbed photons points outwards), $\Delta P_{\rm rad}$ induces a positive pressure gradient (see Figure \[figConstantP\]), whereas gravity must be balanced by a 