PDR models for M8 for G₀ = 10^4.8 and n_H = 10⁵ cm⁻³ (left panel) and G₀ = 10⁵ and n_H = 10⁷ cm⁻³ (right panel). The fractional abundances (n_X∕n_H) are represented by the color codes/lines patterns indicated on the plots and measured by the ordinates. The top abscissa indicates the corresponding visual extinction (A_V) as seen from the ionizing front (the impinging radiation field goes from left to right of the plot). The first bottom abscissa gives the corresponding total hydrogen column density N(H). A second bottom abscissa shows the actual distance (in parsec) from the ionizing front. The vertical dashed lines demarcate the region where most of the hydrogen gas is in molecular form.