THERMAL-PROCESSING OF INTERSTELLAR DUST GRAINS IN THE PRIMITIVE SOLARENVIRONMENT

The heating and vaporization of dust grains in the protosolar environm ent is modeled in order to assess the survivability of interstellar so lids during the formation of the solar system. A multidimensional, dis crete ordinate radiative transfer code is used to compute thermal tran sport in the collapsing protosolar cloud. The results are combined wit h estimates of heating at the shock where infalling material arrives a t the surface of the solar nebula/accretion disk, and in the interior of the disk, to determine the distances at which various solid phases are vaporized. The thermal coupling between the envelope and the accre tion disk (backheating) is treated self-consistently, so its effect on the disk's radial temperature profile is included. This treatment als o permits evaluation of the effect of backheating on the observational inference of disk properties. Calculations are performed for various values of cloud collapse rate, rotation rate, and disk accretion rate. The latter factor is the main determinant of the total luminosity, an d we consider both ''low-luminosity'' cases, in which disk accretion i s inefficient, and high-luminosity'' cases, in which disk accretion ke eps pace with cloud collapse. We also examine situations in which a po lar, optically thin cavity is swept clear by a protosolar wind. We con clude that refractory grains, such as silicates, can generally survive the envelope and accretion shock, and enter the nebula at or within 1 AU. Inside the nebula, their vaporization distances are controlled by the disk accretion rate and optical depth. In contrast, the vaporizat ion distances of volatiles such as water ice are sensitive to envelope conditions, which control the thermal state of the outer, optically t hin regions of the disk. The ice vaporization distance lies between ab out 2 and 30 AU, depending on the total source luminosity and characte ristics of the collapsing cloud. Moderately volatile organics (methano l, formaldehyde, and polymerized formaldehyde) may survive as solids i n the terrestrial planet region; they generally are not vaporized outs ide of several AU, which supports the idea that comets inherit this ma terial from the parent molecular cloud.