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(Abridged) Classification schemes for YSOs are based on evaluating the degreeof dissipation of the surrounding envelope, whose main effects are theextinction of the optical radiation from the central YSO and re-emission in thefar-infrared. Since extinction is a property of column density along the lineof sight, the presence of a protoplanetary disk may lead to a misclassificationwhen the system is viewed edge-on.  We performed radiative transfer calculations, using the axysimmetric 3Dradiative transfer codes RADMC and RADICAL, to show the effects of differentgeometries on the main indicators of YSO evolutionary stage, like the slope ofthe flux between 2 and 24mum, the bolometric temperature and the optical depthof silicates and ices.  We show that for systems viewed at intermediate angles the 'classical'indicators of evolution accurately trace the envelope column density, and theyall agree with each other. On the other hand, edge-on system are misclassifiedfor inclinations larger than ~65deg. In particular, silicate emission, typicalof pre-main sequence stars with disks, turns into absorption when the diskcolumn density reaches 1e22cm-2, corresponding e.g. to a 5e-3 Msun flaring diskviewed at 64deg. A similar effect is noticed in all the other classificationindicators studied alpha, Tbol, and the H2O and CO2 ices absorption strengths.This misclassification has a big impact on the nature of the flat-spectrumsources (alpha ~0), whose number can be explained by simple geometricalarguments without invoking evolution. A reliable classification scheme using aminimal number of observations is constituted by observations of the mm-fluxwith both a single dish and an interferometer.

Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations