Numerical simulations for the northern H ii region. Simulations with and without gravity are in colours and black, respectively. Continuous lines correspond to simulations with similar average initial density, ⟨ ρ_initial ⟩ = 300 cm^-3. The envelope has a decreasing power-law density with the characteristics of the observed one and extrapolated to a constant core of radius 0.005 pc (black) and 0.05 pc (dark blue). For the blue dashed line, the envelope has a power law decreasing density with an index corresponding to the observed gradient but with a 50 cm^-3 density at 1 pc extrapolated to a constant core at 0.005 pc. For the cyan and red dashed curve, the envelope has a constant density of 300 cm^-3 (cyan), corresponding to the average initial density, or a higher 2 × 10⁴ cm^-3 density (red dashed), leading to re-collapse.