2008A&A...486..245C

Determining the evolutionary stage of a Young Stellar Object (YSO) is of fundamental importance to test star formation theories. Classification schemes for YSOs are based on evaluating the degree of dissipation of the surrounding envelope, whose main effects are the extinction of the optical radiation from the central YSO and re-emission in the far-infrared to millimeter part of the electromagnetic spectrum. Since extinction is a property of column density along the line of sight, the presence of a protoplanetary disk may lead to a misclassification of pre-main sequence stars with disks when viewed edge-on. We performed radiative transfer calculations to show the effects of different geometries on the main indicators of YSO evolutionary stage. In particular we tested not only the effects on the infrared colors, like the slope α of the flux between 2.2 and 24µm, but also on other popular indicators of YSO evolutionary stage, such as the bolometric temperature and the optical depth of silicates and ices. We used the axisymmetric 3D radiative transfer codes RADMC and RADICAL to calculate the spectral energy distribution including silicates and ice features in a grid of models covering the range of physical properties typical of embedded and pre-main sequence sources. Our set of models compares well with existing observations, supporting the assumed density parametrization and the adopted dust opacities. We show that for systems viewed at intermediate angles (25°-70°) the ``classical'' indicators of evolution are able to classify the degree of evolution of young stellar objects since they accurately trace the envelope column density, and they all agree with each other. On the other hand, edge-on system are misclassified for inclinations larger than ∼65°±5°, where the spread is mostly due to the range of mass and the flaring degree of the disk. In particular, silicate emission, typical of pre-main sequence stars with disks, turns into silicate absorption when the disk column density along the line of sight reaches 1x10²²cm^–2, corresponding e.g. to a 5x10^–3M_☉ flaring disk viewed at 64°. A similar effect is noticed in all the other classification indicators studied: α, T_bol, and the H₂O and CO₂ ice absorption strengths. This misclassification has a large impact on the nature of the flat-spectrum sources (α≃0), whose number can be explained by simple geometrical arguments without invoking evolution. A reliable classification scheme using a minimal number of observations is constituted by observations of the millimeter flux with both a single dish and an interferometer.