Constraints upon density gradients in evolved HII regions

We investigate the variation of radio flux ratios and brightness temperatures in Galactic HII regions. These are compared to the results of a radiative transfer analysis, and shown to imply that large-scale density gradients must be relatively modest. Where the radial variation in density gradients is approximated by n ∝ r−β, then we determine that exponents β are ⩽1 (and probably ≪1). Similarly, it is probable that proportionate variations in density are relatively modest, and no greater (in the mean) than ∼102 or so. Such results are important for studies of the formation of such regions, and appear consistent with various hydrodynamical models.