Distribution of the core-integrated dust fluxes shown with a histogram (left) and compared to the distrubution of free-free fluxes (right). Left: cores with an HMC emission and/or an H/UC HII region (stages b to d, shown in green) are compared to the “young cores” without evidence of massive star formation (stage a, shown in blue). The vertical lines represent the median value of the distributions. Right: the core sample in this plot is made of the 35 cores with (robust or tentative) H30α detection, classified as cores with an H/UC HII region. The dashed lines represent constant values of the percentage of free-free emission P_ff with respect to the total core flux. Uncertainties were evaluated as 10% of the free-free and B6 fluxes.