2. Observational evidence for crystalline silicates in space
As a basis for the experimental strategy of this paper, we will summarize the overwhelming evidence for the presence of crystalline silicates in space.
Primitive meteorites present convincing evidence concerning the origin of the solar system. Starting from a fine-grained crystalline or amorphous precursor material, the matrix of carbonaceous chondrites has most probably suffered from a thermal evolution influenced by hydrous alteration (Taylor 1992; Brearley 1996). Low-temperature annealing () appears to be the most likely mechanism to reproduce the features of the matrix. Later, the texture and stoichiometry has been changed by aqueous alteration and thermal metamorphism of the meteorite parent body (Kojima & Tomeoka 1996).
Apart from nanometre-sized particles, amorphous bulk material should be present in protoplanetary accretion disks. The prominent objects in chondrites, olivine-pyroxene chondrules that have typical diametres of about 0.5 to 1.5 mm, appear to have crystallized from flashlike molten or partly molten drops (Taylor 1992). Hence, the overall kinetics of bulk crystallization has to be investigated experimentally.
2.2. IDPs and cometary grains
Comets, such as C/1995 O1 (Hale Bopp), are frozen reservoirs of primitive materials from the early solar system and represent a major source for interplanetary dust particles (IDPs) (Sandford 1996; Crovisier et al. 1997; Bradley et al. 1999b). These chondritic porous IDPs contain submicrometre-sized forsterite, enstatite and glassy silicate grains. The glassy silicates are also called GEMS (glass with embedded metal and sulfides). The structural and optical properties of the GEMS are similar to those expected for interstellar silicates and their structure suggests a pre-accretional origin (Bradley et al. 1999a).
Both carbonaceous and silicatic features are present in cometary spectra. In detail, the mid-IR emission spectra of comet P/Halley imply that Mg-rich crystalline olivines are a major mineral component of the inner coma. In addition to crystalline olivines, amorphous Mg-silicates are present in cometary dust (Hanner 1999).
Crystalline silicates in cometary dust have been probably formed by the process of annealing. If stimulation by irradiation and OH-enrichment would not be sufficient to promote crystallization at low temperatures, this fact would imply that solar system formation must have been accompanied by large-scale radial mixing of condensates from the hot terrestrial planet zones to the icy planetesimal zones beyond Jupiter's orbit (Wooden et al. 1999).
2.3. Circumstellar environment around young stars
Amorphous silicates with composition and structure inherited from the parent molecular cloud enter an accretion disk and move slowly inwards. Proceeding to layers of sufficiently high temperature, diffusion and crystallization processes start. At different material-dependent timescales, the amorphous structure is converted into an ordered lattice structure.
In detail, in the shell surrounding the isolated Fe star HD 142527, a warm (500 to 1500 K) and a cold (30 to 60 K) dust component could be distinguished. The cold dust is mainly composed of partially crystalline silicates. Surprisingly, crystalline silicates have not been observed in the warm dust environment (Malfait et al. 1999). As was already noted in the preceding section, crystallization of silicates in cold environments might have been stimulated by irradiation or OH-enrichment. The importance of low-temperature crystallization processes has been outlined recently by Molster et al. (1999b).
Modelling of a thin steady-state protoplanetary accretion disk with a central stellar mass of and an accretion rate of showed that crystallization of the dirty silicate dust starts at about 800 K at a radial distance from the star of 2...1 AU (Gail 1998).
2.4. AGB and Post-AGB stars
In the AGB stage, stars are cool (K), luminous () and extended () red giants. Their mass loss rates range between less than and more than . The central stars are usually obscured by their dusty envelopes. They are detected by the infrared emission from their expanding dust shells. The expansion velocity is in the range from 10 to 25 km . AGB stars are an important dust source injecting newly formed grains into interstellar space ("stardust"). In stars of spectral type M, the oxygen abundance exceeds that of carbon. Hence, M-stars mainly form silicate dust. For reasons of elemental abundance, it is generally believed that the dominating dust condensates are olivine and pyroxene (Dorschner & Henning 1995; Henning 1999).
Silicate condensation may start at a temperature of 1000 K or slightly lower on some preexisting nuclei (e.g. clusters). Silicates should be amorphous at the initial stage of condensation. During the process of annealing, an ordered structure can develop. An at least partially crystalline structure would be established if the cooling rate of the outflow were comparable to the typical annealing time (Gail & Sedlmayr 1998; Gail & Henning 1999).
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000