Infall models of Class 0 protostars

We have carried out radiative transfer calculations of infalling, dusty envelopes surrounding embedded protostars to understand the observed properties of the recently identified “Class 0” sources. To match the far-infrared peaks in the spectral energy distributions of objects such as the prototype Class 0 source VLA 1623, pure collapse models require mass infall rates ∼10−4M yr−1. The radial intensity distributions predicted by such infall models are inconsistent with observations of VLA 1623 at sub-mm wavelengths, in agreement with the results of André et al. (1993) who found a density profile of ρ ∝ r−1/2 rather than the expected ρ ∝ r−3/2 gradient. To resolve this conflict, while still invoking infall to produce the outflow source at the center of VLA 1623, we suggest that the observed sub-mm intensity distribution is the sum of two components: an inner infall zone, plus an outer, more nearly constant-density region. This explanation of the observations requires that roughly half the total mass observed within 2000 AU radius of the source lies in a region external to the infall zone. The column densities for this external region are comparable to those found in the larger Oph A cloud within which VLA 1623 is embedded. This decomposition into infall and external regions is not unique, owing to uncertainty in the structure of the molecular gas outside of the infall zone, which in turn implies some uncertainty in estimating the infall rate. Nevertheless, the environment of Oph A is so dense that any protostellar clouds which fragment out are likely to collapse at very high infall rates, consistent with our spectral energy distribution modeling. The extreme environments of Class 0 sources lead us to suggest an alternative or additional interpretation of these objects: rather than simply concluding with André et al. that Class 0 objects only represent the earliest phases of protostellar collapse, and ultimately evolve into older “Class I” protostars, we suggest that many Class 0 sources could be the protostars of very dense regions, and Class I objects found in lower-density regions may be in comparable evolutionary states.