Infall Models of Class 0 Protostars

We have carried out radiative transfer calculations of infalling, dustyenvelopes surrounding embedded protostars to understand the observed propertiesof the recently identified ``Class 0'' sources. To match the far-infrared peaksin the spectral energy distributions of objects such as the prototype Class 0source VLA 1623, pure collapse models require mass infall rates$\sim10^{-4}\msun$yr$^{-1}$. The radial intensity distributions predicted bysuch infall models are inconsistent with observations of VLA 1623 at sub-mmwavelengths, in agreement with the results of Andre et al. (1993) who found adensity profile of $\rho \propto r^{-1/2}$ rather than the expected $\rho\propto r^{-3/2}$ gradient. To resolve this conflict, while still invokinginfall to produce the outflow source at the center of VLA 1623, we suggest thatthe observed sub-mm intensity distribution is the sum of two components: aninner infall zone, plus an outer, more nearly constant-density region. Thisexplanation of the observations requires that roughly half the total massobserved within 2000 AU radius of the source lies in a region external to theinfall zone. The column densities for this external region are comparable tothose found in the larger Oph A cloud within which VLA 1623 is embedded. Theextreme environments of Class 0 sources lead us to suggest an alternative oradditional interpretation of these objects: rather than simply concluding withAndre et al. that Class 0 objects only represent the earliest phases ofprotostellar collapse, and ultimately evolve into older ``Class I'' protostars,we suggest that many Class 0 sources could be the protostars of very denseregions. (Shortened)Comment: 22 pages, including 3 PostScript figures, accepted for publication in The Astrophysical Journa