Probability density functions for the subregions identified in Perseus where protostars were detected. In each panel, s gives the slope of the power-law fit, shown in red, in the interval delimited by x₁ and x₂, both in 10²¹ cm⁻²; the fitting procedure is detailed in the text. The x-axis is N(H₂) in cm⁻². Green lines extend the best fit over the interval of column densities where protostars were found (see Col. N(H₂) in Table 9).