a) Average number of RMS sources as a function of dynamic age. b) Average apparent (in blue) and physical (in red) surface densities of RMS sources as a function of dynamic age. The error bars show the standard deviation of the mean (±). c) Fragmentation time as a function of dynamic time (Whitworth et al. 1994), assuming c_s = 0.2 (in black) and 0.6 km s^-1 (in blue). The solid red line represents the one-to-one relation.