The IR spectra of silicates in the interstellar medium and around most Asymtotic Giant Branch (AGB) stars show that they are amorphous and characterized by a broad, relatively featureless pair of infrared peaks at 9.7 and 20 microns. Although recent observational evidence has shown that a fraction (10%) of the silicate formed around the highest-mass-loss-rate AGB stellar population is actually crystalline (28, 29), it is unlikely that crystalline grains comprise a significant fraction of interstellar silicates. Even if crystalline silicates escape destruction via supernova shock waves (30, 31), their crystallinity will be destroyed via long exposure to galactic cosmic rays (32). Therefore, the silicate dust observed around most premain sequence (PMS) stars should be amorphous.
If we examine the IR spectra of Herbig Ae/Be stars, we find a mixture of dust types, including highly amorphous silicates akin to interstellar silicates and to glass with embedded metal sulfide (GEMS) within interplanetary dust particles. We also find reasonably well crystallized materials (16, 19, 34, 35). The most conspicuous examples of crystallized materials are seen in the Herbig Ae/Be stars that are either near the zero-age main sequence (ZAMS) and/or exhibit IR excesses suggestive of significant clearing of the inner disk (30, 36-40), including HD 100546, where the silicate emission bands are indistinguishable from those seen in comet Hale-Bopp. Younger and/or less centrally cleared Herbig Ae/Be stars tend to show stronger amorphous silicate emission (16, 41-43).
This trend prompted Waelkens et al. (34) and Nuth to suggest that the silicate crystallinity should increase with stellar age due to processes occurring within the circumstellar disk. The presence of amorphous silicates resembling interstellar grains and the presence of GEMS in interplanetary dust particles effectively eliminates dust processing via an accretion shock as the source of the crystalline material seen in older nebulae. Though Molster et al. (45) argued for low temperature crystallization of dust in higher density regions of disks (based on detection of similar silicate bands in circumstellar material associated with evolved stars that are in the process of building planetary nebulae), no mechanism for such crystallization has been proposed. In contrast, laboratory annealing studies of Mg silicates summarized below provide constraints on the temperature and time required to transform amorphous, circumstellar/interstellar silicate dust into crystalline grains.
Hallenbeck et al. (46) published a study of the evolution of amorphous magnesium silicate smokes subjected to thermal annealing in vacuo at temperatures near 1,000 K. The rate of evolution of the spectrum of silicates is extremely sensitive to the temperatures to which they are exposed. A more recent study can be used to predict the IR spectrum of grains annealed at any given temperature and time (47). Whereas magnesium silicate smokes anneal beyond the stall (46) in only 2 h at 1,048 K, at 1,000 K this same transition requires 300 days. Annealing small interstellar silicates at significantly higher temperatures (e.g., T > 1,500 K) could result in their vaporization, whereas annealing at lower temperatures could require millions of years to achieve significant changes (Table 1), and thus cannot be the source of crystalline silicates in young Herbig Ae/Be stars. Malfait et al. (36) demonstrated the similarity of the dust spectrum for HD 100546 (t = 10 million years) to dust in comet Hale-Bopp, and Knacke et al. (48) have suggested similarities between dust in the Pictoris system and that seen in olivine-rich comets. More recent observations (49) suggest that dust in this system is a mixture of amorphous silicates and only a small fraction of crystalline grains. Because dust around Pictoris, and potentially also around HD 100546 (22) most likely originates in evaporating comets, the dust in those comets must have been processed at high temperatureson the order of 1,000 K. And because Comets Halley, Hale-Bopp, Bradfield, and Mueller also exhibit olivine-rich dust features (50), one can infer that some dust in these comets was also exposed to high temperatures before incorporation into the individual comet. Hallenbeck et al. (46) demonstrated that dust spectra of these comets can be fit by using the same set of IR peaks used to fit the spectra of partially annealed magnesium silicate smokes in the laboratory (Fig. 2). Given the similarity of the laboratory spectra to those in comets, we believe that we understand the degree of annealing required in the natural system to reproduce the observations. Such temperatures (1,000 K) are only found within the innermost regions of the solar and other protoplanetary nebulae (51).