RADIATIVE HEATING OF INTERSTELLAR GRAINS FALLING TOWARD THE SOLAR NEBULA - 1-D DIFFUSION CALCULATIONS

As the dense molecular cloud that was the precursor of our Solar Syste m was collapsing to form a protosun and the surrounding solar-nebula a ccretion disk, infalling interstellar grains were heated much more eff ectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temper atures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cy lindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent mode ls for the composition and radiative properties of interstellar grains (Pollack ef al. 1994. Astrophys. J. 421, 615-639), thereby allowing u s to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rota tion rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grai n-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of th e infalling grain species, even the refractory silicates and iron, wil l vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice spe cies, even volatile organics, will survive intact to the disk accretio n shock. These grain-survival conclusions are subject to several limit ations which need to be addressed by future, more sophisticated radiat ive-transfer models. Nevertheless, our results can serve as useful inp uts to models of the processing that interstellar grains undergo at th e solar nebula's accretion shock, and thus help address the broader qu estion of interstellar inheritance in the solar nebula and present Sol ar System. These results may also help constrain the size of the accre tion disk; for example, if we require that the calculations produce pa rtial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or gr eater than a few AU. Intriguingly, this is comparable to the heliocent ric distance that separates the C-rich outer parts of the current Sola r System from the C-poor inner regions. (C) 1997 Academic Press.