FLAT SPECTRUM T-TAURI STARS - THE CASE FOR INFALL

We show that the mid- to far-infrared fluxes of ''flat spectrum'' T Ta uri stars can be explained by radiative equilibrium emission from infa lling dusty envelopes. Infall eliminates the need for accretion disks with nonstandard temperature distributions. The simplicity and power o f this explanation indicates that models employing ''active'' disks, i n which the temperature distribution is a parameterized power law, sho uld be invoked with caution. Infall also naturally explains the scatte red light nebulae detected around many flat spectrum sources. To match the observed spectra, material must fall onto a disk rather than the central star, as expected for collapse of a rotating molecular cloud. It may be necessary to invoke cavities in the envelopes to explain the strength of optical and near-infrared emission; these cavities could be produced by the powerful bipolar outflows commonly observed from yo ung stars. If viewed along the cavity, a source may be lightly extinct ed at visual wavelengths, while still accreting substantial amounts of material from the envelope. Infall may also be needed to explain the infrared-bright companions of many optical T Tauri stars. This picture suggests that many of the flat spectrum sources are ''protostars''-yo ung stellar objects surrounded by dusty infalling envelopes of substan tial mass.