Evolutionary stages of high-mass star formation from the observational viewpoint
Unlike their low-mass siblings, high-mass (OB-type) stars reach the zero-age main sequence still deeply embedded in their parental clumps. This makes it difficult to establish the evolutionary stage of a massive young stellar object (YSO) from the shape of the spectral energy distribution (SED). However, in recent years the sensitivity and resolution of observations at mid- and far-IR wavelengths have undergone substantially improvement, as well as the radiative transfer models of circumstellar envelopes. These have made it possible to classify high-mass star forming regions on the basis of their SEDs. In particular, I will show how the source morphology and SED have been used to estimate the evolutionary stage of high-mass YSOs. I will also discuss additional observational evidence that can be used to complement the SED-based classification, such as HII regions, maser emission, outflows, and other tracers of the structure and kinematics of the molecular envelope enshrouding the high-mass YSOs. The results will be illustrated by means of template objects in different evolutionary phases and the role of the new generation instruments (Herschel, ALMA, EVLA) will be outlined.