The evolution of the spectral energy distribution in massive young stellar objects

Context. The mechanism of formation of massive stars is still a matter of debate. It is not yet clear if it can be considered to be a scaled-up analogue of the low-mass star regime, or if there are additional agents like merging of lower-mass forming objects or accretion from initially unbound material. Most of the uncertainties come from the lack of diagnostic tools to evolutionarily classify large samples of candidate massive protostellar objects that can then be studied in more detail. Aims. We want to verify whether diagnostic tools like the SED shape and the relationship between envelope mass and bolometric luminosity can be extended to the study of high-mass star formation. Methods. The 8-1200 mu m SED of YSOs in 42 regions of massive star formation has been reconstructed using MSX, IRAS, and submm data partly available from previous works. Apart from IRAS catalogue fluxes, the fluxes in the Mid-IR and sub-mm/mm were derived directly from the images. The SEDs were fitted to an extensive grid of envelope models with embedded ZAMS stars, available from the literature. Sources that could not be fitted with a single model were then fitted with a two-component model composed of an embedded ZAMS for the mid-IR part and a single-temperature optically thin greybody for the longer wavelength emitting component. Sources were classified as IR if they were fitted with an embedded ZAMS envelope, and MM if they could only be fitted with a greybody with a peak at high.; further subclassification was based on being the most massive object in the field (P, for primary) or not (S, for secondary). Results. The different classes of sources identified in our analysis have very different SEDs and occupy distinct areas in the L-bol-M-env diagram; by analogy with the low-mass regime, we see that MM-P, IR-P and IR-S objects could be interpreted as the high-mass analogue of Class 0-I-II. Evolutionary tracks obtained from a simple model based on the turbulent core prescriptions show that the three classes of sources possibly mark different periods in the formation of a massive YSO. The IR-P objects are consistent with being at the end of the main accretion phase, while MM-P sources are probably in an earlier evolutionary stage. The timescales for the formation decrease from similar to 4 x 10(5) to similar to 1 x 10(5) years with stellar mass increasing from similar to 6 to similar to 40 M-circle dot; these timescales, and the association with young clusters with median stellar age of a few 10(6) years suggest that the most massive objects are among the last ones to form. Conclusions. Our results are consistent with the high-mass star formation being a scaled-up analogue of the traditional accretion-dominated paradigm valid for the low-mass regime.