Envelope/core mass with respect to the bolometric luminosity. For the massive protostars studied here, in light blue circles, the values are as estimated from the SED fittings (see Sect. 4.1 and Appendix A for more details). The remaining filled and empty symbols are the low-mass Class I and Class 0 objects, respectively (from Bontemps et al. 1996 in black; and from Motte & André 2001; and André et al. 2000, in dark blue). The dotted curves show four examples of evolutionary tracks with decreasing accretion rates, for envelopes with initial masses of 0.6, 2, 8, and 20 M_⊙ (see Sect. 6). The arrows show the positions in each track where 50% and 90% of the envelope mass has been accreted onto the protostar. The dashed line represents the relation. The placement of the curve at comes as an empirical border line between Class 0 and Class I evolutionary stages.