The structure of protostellar envelopes derived from submillimeter continuum images

High dynamic range imaging of submillimeter dust emission from the envelope s of eight young protostars in the Taurus and Perseus star-forming regions has been carried out using the Submillimeter Common-User Bolometer Array (S CUBA) on the James Clerk Maxwell Telescope. Good correspondence between the spectral classifications of the protostars and the spatial distributions o f their dust emission is observed, in the sense that those with cooler spec tral energy distributions also have a larger fraction of the submillimeter flux originating in an extended envelope than in a disk. This results from the cool sources having more massive envelopes rather than warm sources hav ing larger disks. Azimuthally averaged radial profiles of the dust emission are used to derive the power-law index of the envelope density distributio ns, p (defined by p proportional to r(-p)), and most of the sources are fou nd to have values of p consistent with those predicted by models of cloud c ollapse. However, the youngest protostars in our sample, L1527 and HH 211-m m, deviate significantly from the theoretical predictions, exhibiting value s of p somewhat lower than can be accounted for by existing models. For L15 27 heating of the envelope by shocks where the outflow impinges on the surr ounding medium may explain our result. For HH 211-mm another explanation is needed, and one possibility is that a shallow density profile is being mai ntained in the outer envelope by magnetic fields and/or turbulence. If this is the case, star formation must be determined by the rate at which the su pport is lost from the cloud, rather than the hydrodynamical properties of the envelope, such as the sound speed.