Numerical simulations for the central UCH ii region expansion. In black: without gravity and in an envelope of decreasing density, which is extrapolated from the observed one up to a constant density within r_c = 0.005 pc. In dotted black: without gravity in an envelope of decreasing density with a gradient typical of infall, ρ ∝ r^-1.5. In cyan: with gravity and in an envelope with a constant density of 1.65 × 10⁵ cm^-3, just allowing the observed size to be reached (black dashed line). In dashed red: with gravity and in an envelope with a constant density of 2.5 × 10⁵ cm^-3, collapses again before reaching the observed size.