"Rainfall" from protostellar envelopes onto protoplanetary disks
Class 0 protostars, the youngest type of young stellar objects, show many signs of rapid development from their initial, spheroidal configurations. They are therefore studied intensively for details of the formation and nature of the dense cores within protostellar envelopes. At millimeter wavelengths, kinematic signatures of envelope collapse have been observed in several such objects. These objects and many more Class 0s also show evidence of strong high-velocity bipolar outflows. The long-sought link between these two flows and the central protostar -- the embedded protoplanetary disk -- has recently been discovered in observations with the Spitzer Space Telescope's Infrared Spectrograph. We have detected rich emission spectra of water vapor --in the three most abundant isotopologues -- and hydroxyl, at wavelengths 20-37 microns, in ten Class 0 objects in the NGC 1333, Orion A and Chamaeleon II clouds. The observations indicate directly the presence of extremely dense, warm gas from regions of Solar-system dimension. Models based upon vigorous infall onto a disk, in the form of a plane-parallel disk-accretion shock, reproduce the observed spectra well. More complex models will enable precise constraints to be placed on the structure and stability of the youngest protoplanetary disks. The observations also show directly that water arrives in protoplanetary disks as a warm vapor rather than ice, thus erasing any chemical signature of this material's interstellar origins.
