PROTOSTELLAR COLLAPSE IN A SELF-GRAVITATING SHEET

We present preliminary calculations of protostellar cloud collapse sta rting from an isothermal, self-gravitating gaseous layer in hydrostati c equilibrium. This gravitationally unstable layer collapses into a fl attened or toroidal density distribution, even in the absence of rotat ion or magnetic fields. We suggest that the flat infalling envelope re cently observed in HL Tau by Hayashi et al. is the the result of colla pse from an initially nonspherical layer. We also speculate that the l ater evolution of such a flattened, collapsing envelope can produce a structure similar to the ''flared disk'' invoked by Kenyon and Hartman n to explain the infrared excesses of many T Tauri stars.