Influence of the adopted structure for the champagne flow on the derived values of the radiation pressure efficiency, . Plotted is versus the scale height of the flow, R₀, normalized to the distance star – cloud, R_s. The dashed line marks the scale height, corresponding to a flat ionization front, and is the one assumed in our calculations. Smaller values of R₀ correspond to increasingly diverging flows, characterized by a fast acceleration of the gas. Larger values of R₀ correspond to thick flows that accelerate more slowly. Plotted is such that the equilibrium radius equals r_eq = 1.0 (solid lines) and r_eq = 0.1 (dashed lines). The upper curves correspond to an ionizing flux of log (Q₀) = 48.25 photons s^-1 (Schaerer & de Koter 1997), the lower curves correspond to an ionizing flux of log (Q₀) = 47.56 photons s^-1 (Martins et al. 2005).