Sheet Models of Protostellar Collapse

Recognizing that protostellar clouds are unlikely to be completely spherical, we explore some effects of initial cloud geometry by considering collapse from a sheet initially in hydrostatic equilibrium. A qualitatively different feature of sheet collapse compared with spherical contraction is the development of relatively evacuated cavities in the infalling dusty cloud, which arise because material falls in first along the shortest dimension to the central gravitating mass. Using analytic models of collapse, which reproduce the main features of our previous numerical time-dependent simulations, we perform detailed radiative transfer calculations, which suggest that these collapse cavities can naturally explain the morphological appearance of many reflection nebulae around young stars on small distance scales without requiring initially diverging outflows. Sheet collapse models can simultaneously explain small-scale reflection nebula morphologies and dust envelope emission properties of many young stellar objects more easily than the standard 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 visible over a substantial range of system inclinations to the line of sight. These results may be especially relevant to cases where fragmentation 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 star HL Tau can be interpreted in terms of flattened protostellar cloud collapse and draw some distinctions between the flattened toroids resulting in our calculations and the "pseudodisk" of Galli & Shu.