Notes. Columns 2−5 describe the density profile of the neutral envelopes surrounding H ii regions by listing the radius separating the inner and outer envelope components, the outer radius, and the density power law slopes of the two envelope components. Uncertainties (3σ) on q_out are estimated to be 0.2. Column 6 gives the time elapsed since the beginning of infall and protostellar collapse. It is calculated from R_Infall (Col. 2) and Eq. (11) with a velocity range from one to three times the sound speed (see Sect. 5.2) and has a statistical error of 20%. Columns 10−11 give the absolute values of the density measured at r = 1 pc and at the R_{H ii} radius, from the complete modelling of column density profiles (see Sect. 3.5). Column 11 is used to estimate, through Eq. (4), the mean initial neutral gas density before the ignition of an H ii region.