Core luminosity versus core mass. We make four categories of sources: “near” sources, which are closer than 4 kpc, with either a 24 μm counterpart (“bright”) or no 24 μm counterpart (“dark”), and “far” sources with the same bright/dark distinction. For comparison, we plot in orange triangles the Beuther et al. (2002b, 2005a) sample of high-mass protostellar objects (HMPOs) and the Hunter et al. (2000) sample of ultra-compact HII regions in pink squares. With the black dashed line, we plot a line of constant accretion luminosity, L_acc = GṀM_star/R_star, of 10^-5   M_⊙ yr^-1, assuming M_star = 0.1   M_core and R_star = 5   R_⊙, and the range indicated by the grey dashed lines on either side show  ± 1 dex variation. See Sect. 6.2 for details.