THE TRANSPORT OF THERMAL-RADIATION IN A PROTOSTELLAR ENVELOPE

We present a discrete ordinate solution for gray radiation transport i n axisymmetric protostellar envelopes, for the purpose of defining the patterns of interstellar-grain survival during the formation of the s olar system. The gray transfer problem is nonlinear because the opacit y depends on temperature, with discontinuities at temperatures where v arious species of dust vaporize. The standard lambda iteration techniq ues that are required for accurate solutions to the transfer equation tend to be nonconvergent under these conditions. We show that accuracy can be achieved through a relaxation method. We first compare the the rmal profiles in spherically symmetric envelopes computed by the discr ete ordinate solution with those predicted by the diffusion approximat ion. The more accurate discrete ordinate solutions tend to yield steep er temperature gradients, and the central vaporized cavity around the protostar is larger than that given by the diffusion approximation. Th e transport solution is then applied to an axisymmetric model envelope in which the cloud is flattened due to rotation, and in which a wind evacuates the polar regions of the cloud. The resulting cavity beams t he emergent intensity in the polar direction. When the cloud is unifor mly opaque, the polar cavity enhances the diffusive escape of radiatio n and globally reduces temperatures in the cloud. Modeled temperature profiles are used to predict the radial distances at which various dus t, species survive infall in the cloud. The results indicate that surv ival boundaries range from within 1 AU for the most refractory solids, to several AU for volatile organics, and that water ice is excluded f rom within 20-30 AU of the protostar during the collapse phase. We com pare dust vaporization due to heating in the cloud with destruction in the accretion shock; based on the test case, heating in the cloud dur ing collapse is potentially more destructive.