ISM phases. More importantly, this kind of models offers some quantitative insight on the role played by the different physical mechanisms that may be at stake. Although the present study is mostly concerned with gravity, we will also explore the influence of the spectral energy distribution of the incident radiation, as well as the presence of an external potential and a magnetic field.

The role of gravity
-------------------

To illustrate the effect of gravitational acceleration on the equilibrium structure of a generic photoionized region, we ran a first model where the incident continuum consists of the cosmic microwave background and the interstellar radiation field, interpolated from the measurements of @Black87 by means of the [[table ISM]{}]{} command.

![ Density of the different Hydrogen phases (top), electron temperature (middle) and total pressure (bottom) in the standard [[constant pressure]{}]{} case. []{data-label="figConstantP"}](figs/Fig3.eps){width="50.00000%"}

Results for different values of the central density $n_0$ are plotted in Figure \[figN\]. In all cases, there is an inner region where the gas is almost fully ionized, a thin layer of molecular and atomic Hydrogen at high densities, and an outer region of low-density gas where Hydrogen is predominantly neutral, becoming more and more ionized at large distances as 