ON THE VALIDITY OF GREY RADIATION TRANSFER IN DUSTY ENVELOPES

Radiative transfer calculations in dusty protostellar envelopes have b een performed to test the validity of the grey approximation which is often used in radiation hydrodynamic calculations. In the spherical sy mmetric (1D) case we compare grey calculations with the frequency depe ndent exact solution of radiation transport. In the 2D case we use the flux-limited diffusion approximation to solve the radiation transport equation in grey and frequency dependent form. In all cases the dust temperature distributions are determined self-consistently. Model enve lopes are considered with a density peaked structure rho proportional to r(-alpha) and with different total optical depths in the 1D case. I n the 2D case we use the density distributions of various disk models. A dust model consisting of amorphous carbon and silicate grains, part ially coated with ''dirty ice'', is assumed. We find that the dust tem peratures are generally underestimated in the grey approximation (by f actors up to three) and that the radiative acceleration can be underes timated by orders of magnitude. The importance of efficient but more a ccurate algorithms than the usual grey approximation is stressed.