Anisotropic Evaporation of Forsterite and Its Implication for Dust Formation Conditions in Circumstellar Environments

Optical constants of refractory oxides at high temperatures

“…Allowing for a range of axis ratios around a somewhat changed value of r c /r a = 0.70 would give the right feature (cf. Takigawa et al 2012; see also Bohren & Huffman 1983, Fig. 12. An average profile of the astronomical 13 μm emission band -as derived by Fabian et al (2001) for a sample of 23 oxygen-rich AGB stars -compared to the normalized emission spectra of spherical corundum grains at temperatures of 300 K to 928 K. Fig.…”

“…As shown by Posch et al (1999), the following dilemma arose, based on the room temperature data: a) for small spherical particles, corundum would produce a 12.7 μm feature with too narrow a bandwidth; b) for a continuous distribution of ellipsoids, α-Al 2 O 3 would produce a 13.2 μm feature with an up to 7 times too large bandwidth. Takigawa et al (2012) proposed to solving this dilemma by introducing ellipsoidal grains of a particular shape. In their condensation experiments, they found that the condensation rates of crystalline Al 2 O 3 are quite different for the different crystallographic axes, which leads to a flattening of the condensed particles along the crystallographic c-axis.…”

“…Broader band profiles may indicate higher (mean) dust temperatures. Figure 13 refers to oblate spheroidal grains with the experimental axis ratio found by Takigawa et al (2012), i.e. r c /r a = 0.79.…”

Absorption Efficiencies of Forsterite. I. Discrete Dipole Approximation Explorations in Grain Shape and Size

“…Hence, the asymmetric tri-axial shapes of forsterite crystals are important shape characteristics with respect to spectral features' shape, peak position, and relative strength. DDA computations allow explorations of asymmetric shapes or "crystallographically anisotropic shape" (Takigawa et al 2009). Since tri-axial asymmetric shapes probably exist in lab samples and astronomical sources, the varying resonant features' sensitivities to the asymmetric renditions of the brick highlight the need to account for the asymmetric tri-axial nature of forsterite in modeling and interpreting observational data.…”

“…Thus, the grain shape of the crystals must be considered in parallel to external grain composition when modeling the dust in comet comae, especially when the derivation of f cryst is sought. The shape of the crystals in comet dust could be indicative of the process of crystal formation by either gas phase condensation (Tsuchiyama 1998;Bradley et al 1983;Kobatake et al 2008), selective evaporation (Tsuchiyama & Tachibana 1998;Takigawa et al 2009), or annealing (Koike et al 2006(Koike et al , 2010. Furthermore, if variations in f cryst are used to interpret radial transport efficiencies or dust evolution processes in PPDs (e.g., Manoj et al 2011;Hughes & Armitage 2010), accurate model descriptions of the dust, including the crystalline component, are required (Olofsson et al 2009;Oliveira et al 2011).…”