Metamorphism of Cosmic Dust: Diagnostic Infrared Signatures

Extract

Nucleation is a non-equilibrium process: the products of this process are seldom the most thermodynamically stable condensates but are instead those which form fastest. It should not be surprising that grains formed in a circumstellar outflow will undergo some degree of metamorphism if they are annealed or exposed to a chemically active reagent. As a consequence of this processing in the laboratory one observes a continuous increase in the strength of the silicate absorption band at 20 microns relative to the 10 micron feature. In Section 1 we show that this ratio can be used as an indicator of the relative age of silicate condensates. Metamorphism of refractory particles continues in the interstellar medium (ISM) where the driving forces are sputtering by cosmic ray particles, annealing by high energy photons and grain destruction in supernova generated shocks. Studies of the depletion of the elements from the gas phase of the ISM tell us that if grain destruction occurs with high efficiency, then there must be some mechanism by which grains can be formed in the ISM. Laboratory studies of such a process (Moore, Tanabe, and Nuth, Ap. J. (Lett) 373, L31-L34, 1990) have shown that the frequency of the -SiH stretch can be used as an indicator of the oxidation state of the silicon in such grains. Highly reduced grains exhibit an SiH absorption near 2100 cm⁻¹ whereas highly oxidized silicates absorb near 2300 cm⁻¹: this point is discussed in Section 2.