SHEET MODELS OF PROTOSTELLAR COLLAPSE

Recognizing that protostellar clouds are unlikely to be completely sph erical, we explore some effects of initial cloud geometry by consideri ng collapse from a sheet initially in hydrostatic equilibrium. A quali tatively different feature of sheet collapse compared with spherical c ontraction is the development of relatively evacuated cavities in the infalling dusty cloud, which arise because material falls in first alo ng the shortest dimension to the central gravitating mass. Using analy tic models of collapse, which reproduce the main features of our previ ous numerical time-dependent simulations, we perform detailed radiativ e transfer calculations, which suggest that these collapse cavities ca n naturally explain the morphological appearance of many reflection ne bulae around young stars on small distance scales without requiring in itially diverging outflows. Sheet collapse models can simultaneously e xplain small-scale reflection nebula morphologies and dust envelope em ission properties of many young stellar objects more easily than the s tandard spherical collapse models. The sheet collapse picture suggests that protostars, i.e., young stellar objects still accreting a large fraction of their mass from infalling envelopes, may be optically visi ble over a substantial range of system inclinations to the line of sig ht. These results may be especially relevant to cases where fragmentat ion and collapse has been triggered by an external impulse, such as a shock wave. We show how many properties of the flat-spectrum T Tauri s tar HL Tau can be interpreted in terms of flattened protostellar cloud collapse and draw some distinctions between the flattened toroids res ulting in our calculations and the ''pseudodisk'' of Galli & Shu.