Column density of the atomic hydrogen (v_lsr = 60 − 120 km s^-1) as a function of the total hydrogen column density (N_{H i} + N_H2). The column density for the envelope (r> 140 pc, N_{H i} = 107 ± 20 M_⊙ pc^-2, N_H2 = 116 ± 18 M_⊙ pc^-2) is subtracted. The solid and dashed lines describe the theoretical model of Krumholz et al. (2008, 2009) for solar values (G₀ = 1, Z = 1) and for more realistic values of W43 (G₀ = 5, Z = 1.4), respectively. The blue and black colors represent the model parameters at different equivalent radii of r = 40 and 140 pc, respectively. The black dotted line shows the fitted curve of the elliptical annuli analysis introduced in Sect. 5.3.