Positions of the YSOs in the HRD with well-constrained temperatures and luminosities. Red rectangles outline the parameter space of IRS5A (the upper one) and B273A (the lower one), and the red asterisk at upper left marks UC1. For IRS5A and B273A, the conversion from spectral types to effective temperatures and bolometric luminosities is referred to Sect. 4.5, while UC1 is assumed to be a ZAMS B0.5 star in light of the HCHII region (N+07 and references therein). Left panel: positions of the YSOs compared with the model of massive star formation via disk accretion (H+10). The symbols along the evolutionary loci at various accretion rates and the ZAMS locus denote the mass at each position. Right panel: positions of the YSOs placed against the PMS evolutionary tracks of different masses (CA+12). Isochrones of five ages are drawn as dashed lines in different colors.