Luminosity–mass diagram of target sources (three-branched triangles). The evolutionary tracks of massive clumps of different envelope masses that form a cluster of stars with different accretion rates are shown as dotted and solid green lines; the evolutionary tracks of clumps of different envelope masses that are assumed to form a single massive star are shown as dash-dotted gray lines with arrows (Molinari et al. 2008; for more details see Sect. 2.1).