Sketch of the method adopted to estimate the HII region radius for a generic point P. The contour corresponds to the 5σ level of the 3 mm continuum emission and is assumed to be the border of the HII region, while B is the barycentre of this contour. I is the intersection between the half-line originating in B and passing through P. The segment BI is the radius used in our model calculations to determine the intensity of the hydrogen recombination lines in P. The 0,0 position is the phase centre of the ALMA observations.