Cumulative fraction of sources as a function of the background visual extinction. The top left frame compares starless (green) and protostellar (orange) sources. Solid lines are for all sources, dashed lines are for cores with α_BE< 2. The top right frame shows, for all sources, the number of sources per bin of background column density and the mean fraction of starless (SL, green line) and protostellar (PS, orange line) sources in each bin. The bottom left frame shows the values of visual extinction A_V at 50% of cumulative fraction for different bins of distance. Solid black lines are for all sources, red dashed lines for cores with α_BE< 2. The bottom right frame shows the cumulative fractions for sources with α_BE< 2 obtained per bin of field 9th decile temperatures T_90%. The insert shows the variations in visual extinction A_V at 50% of cumulative fraction as a function of T_90%. The orange solid line shows Eq. (7) for β = 2, α = 4.5, and T_th = 12 K without fitting the data. The error bars in the insert and in the bottom left frame are proportional to 1/, where N_sources is the number of sources in the bin of T_90% or distance. It assumes that the uncertainty on the A_V estimate of each source is 100%.