Report of the Working Group on Silicates

Abstract

There are at least two ways to understand the nature of grains in the universe. In one approach, one can try to understand the life history of typical particles. The fate of a grainmay e.g. be as follows: vapours of refractory compounds are synthesized in the depthsof stars and ejected into circumstellar regions by processes which are not yet well understood. In the circumstellar shell, refractory vapors nucleate into grains which continueto accrete additional material and may coagulate into larger aggregates. While still inthe shell, grains can be annealed at temperatures on the order of 1000 K for timescales ofup to a year. Both, within the circumstellar outflow and the interstellar medium (ISM),interaction of the grain with energetic particles could produce considerable change in thecomposition and structure of the particle. In ISM shock processes (including sputtering,heating and grain-grain collisions) can again alter or even destroy typical grains. Atomsfrom destroyed grains can recondense on grain cores which survived the shock, probablyas SiH₄, Fe(CO) _x , Al(OH), MgH along with the more abundant molecules such as CO,H2O, NH3. UV or cosmic ray processing of these ices could lead to a refractory metal oxide or organo-metallic residue over silicate or metal-oxide grain cores. At some point in agrain’s life it evolves part of a collapsing molecular cloud which eventually evolves into astar. Of the grains in this collapsing cloud, a small percentage survive to from solid bodies such as planets, asteroids or comets; in a few of these cases the grains are sufficientlyrobust or the processing is sufficiently gentle that particles survive this processing intactand retain clues to the environments in which they previously existed.