Relation between derived molecular hydrogen densities and visual extinctions for the sample used in this study. Colors indicate the different observed regions as shown in the plot legend. Densities <3 × 10³ cm⁻³ are only measured in TMC 1 and Barnard 1b as a consequence of our methodology and the limitations of our dataset, which is incomplete for the rest of the regions (see text). This value is marked with a dotted horizontal line.