Interstellar Silicate Mineralogy

Laboratory Astrophysics group of the AIU Jena


Introduction
Silicatic particulates are an ubiquitous component of cosmic dust.
The interplanetary space is populated by a wide size spectrum of small particles that have been produzed by impacts, collisions and disintegration of small bodies. As a rule, solar system silicate dust, e.g. cometary grains and dust around main sequence stars contains crystalline silicates.
In contrast to this, most of the observed silicate bands in spectra of circumstellar or interstellar IR sources have very broad and almost structureless profiles.
But recently, infrared spectra gained by the Infrared Space Observatory (ISO) satellite gave evidence for the presence of crystalline silicates around young and evolved stars and planetary nebulae.

Dust in molecular clouds
Molecular clouds that are the places of star formation are assumed to be the coldest environments in space. Silicate particles that consists of a mixture of amorphous olivine and pyroxene are most probably covered with carbonaceous or organic material. The outermost layer of the particles consists of ice.
Silicate particles in this environment are assumed to be strictly amorphous due to the amorphization by cosmic rays. Infrared spectra of compact molecular sources show the presence of a strong, broad diffuse band centered about 10 micrometer and a weaker , broad diffuse band centered near 18 - 19 micrometer. No crystalline silicate bands are observed.

Interstellar Dust
Interstellar dust is multiform because it consists of particles from a wide range of sources (e. g. formed in stellar atmospheres, novae or supernovae). In the interstellar space it is subjected to the action of high energy rays. It is assumed that this will lead to amorphization of the material. This is also true for silicates.

Dust in accretion disks
The silicates in outer regions of accretion disks are very similar to those in the interstellar medium (ISM). Olivine and pyroxene are the dominant silicates with olivine having a larger mass fraction than orthopyroxene.
In the warmer regions of accretions diskas as well as in the warmer regions of molecular cloud cores, some or all of the grain spezies are chemically  transfomed or sublimate. Silicates change into the crystalline state due to the effect of thermally annealing.
Spiraling inward towards the protostar the grain spezies start to vaporize. Model calculations of solar type stars indicate that silicate vaporation takes place at 1300 ... 1450 K. Thios temperature is reached in the protoplanetary disk at about 0.6 ... 0.3 AU [Gail 1998].

Dust in stellar outflows of oxygen-rich giants
High temperature condensation:
The silicate condensation sequence starts with the formation of Al2O3 which, upon cooling, reacts with gaseous SiO, Ca and Mg to form spinel (MgAl2O4), melilithe and then diopside (CaMgSi2O6). Only a small fraction of the Mg and Si is involved in this part of the condensation sequence.
Olivin and pyroxen condensation:
Most of the silicon nucleates and condenses as forsterite. Possibly the excess SiO converts forsterite into enstatite. Gaseous Fe may react with enstatite grains to form fayalite, rather than condense as metallic iron [Tielens 1998].
Furthermore, the chaotically condensed silicates might be subdued to thermally annealing to transform into the crystalline state.
Generally, in the outflows and ejecta of evolved stars the largest number of different dust spezies among the cosmic dust populations has been found [Dorschner 1999].
Finally, dust condensed in stellar atmospheres becomes part of the interstellar medium.

Meteorites and interplanetary and cometary dust
Interplanetary dust particles (IDPs) have been produced by impacts, collisions and disintegration of small bodies.
But also refractrory grains frozen in cometary nuclei, which are released by the sublimation of the ices through the solar radiation contribute to the IDP population. Fortunately, IDPs can be collected by planes and studied in the laboratory. In such grains hints for presolar components have been found.
Primitive material can also be found as inclusions in chondritic meteorites. These calcium aluminium inclusions (CAI) are strongly suspected of being very primitive, they have been correlated with the first, i.e. highest temperature, condensation products predicted to form from a hot and cooling solar nebulae.
As a rule, even primitive solar system solids IDPs, CAI in chondrites) and the stardust grains isolated form meteorites are crystalline. Furthermore, even infrared observations of comet hale bopp proved that silicate particles of comets that are released by ice-evaporation into the interplanetary space are at least partially crystalline [Croivisier 1999].
This fact is surprising because it was generally assumed that comets formed from rather unprocessed material, i. e. silicates woud be still in amorphous state.
There are long range mixing processes in the solar nebulae neccesary to explain the transport of crystalline silicates from approx. 1 AU to beyond 40 AU!



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Last modified: Dirk Fabian, February 99