Silicates thermal annealing

Apai et al. (2005) report prominent crystalline silicate emission peaks in five out of six, approximately co-eval ( 2 Myr-old), brown dwarf disks. These brown dwarf disks may have higher crystallinity than most disks yet are observed around Sun-like and intermediate-mass stars. Brown dwarfs, being cooler and less luminous than their more massive analogs, can thermally anneal silicates only in a very small inner disk (< 0.05 AU), which is the reason why no crystalline silicates were expected around them. In recent detailed studies of two 1 Myr-old brown dwarf candidates Merfn et al (2007) and Bouy et al. (2008) also found high crystalline mass fractions (15% to 33%). 

The depletion in FeO may be understood in at least two ways. First, the crystalline grains may be equilibrium condensates from a hot solar nebular composition gas with iron sequestered to metals or sulfides (see e.g. Chapter 4). In this case the condensed grains either had to condense slowly to form crystal domains, or had been reheated and thermally annealed at a later epoch. The second, alternative explanation is that ferromagnesian amorphous silicate grains were thermally annealed in a reducing environment, e.g. in the presence of carbon. Heating such precursors leads to the formation of metallic spheroids embedded between the forsterite crystals, as the initial FeO component is reduced (see e.g. Fig. 8.3 and Connolly et al. 1994 Jones Danielson 1997 Leroux et al. 2003 Davoisne et al. 2006). Because carbon is ubiquitously present in primitive Solar System materials, this pathway offers a natural explanation to the observed FeO-poor silicate crystals. It is yet to be determined whether low-temperature crystallization processes, discussed in Section 8.1.1, would also lead to FeO depletion. 

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