Crystallographically anisotropic shape of forsterite: New probe for evaluating dust formation history from infrared spectroscopy
Crystallographically anisotropic shape of forsterite: New probe for evaluating dust formation history from infrared spectroscopy
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镁橄榄石的晶体各向异性形状：通过红外光谱评估灰尘形成历史的新探针
DOI：
10.1088/0004-637x/750/2/149
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发表时间：
2012
影响因子：
4.9
通讯作者：
S.
中科院分区：
文献类型：
--
作者：
Takigawa;A.;Tachibana;S.
红外光谱已经在被尘埃笼罩的渐近巨星分支恒星、原行星盘和一些彗星周围观察到了结晶尘埃。晶体材料往往具有与特定晶体学取向（晶体学各向异性形状）相关的特定形状，反映了晶体的各向异性性质，其红外光谱特征取决于晶体学各向异性形状。因此，晶体各向异性形状是定量评估星周尘埃形成条件的潜在强大探针。为了评估从红外光谱确定晶体各向异性形状的可能性，我们计算了椭圆体镁橄榄石颗粒（最丰富的星周结晶硅酸盐）的质量吸收系数，在 9-70 μm 波长范围内，沿着晶体 a、b 和 c 轴拉长和变平，具有不同的长宽比。研究发现，在 9-12 μm 和 15-20 μm 范围内，无论温度、尺寸、化学成分和镁橄榄石晶粒边缘的影响，不同晶体各向异性形状引起的红外特征差异都是可区分的。因此，我们得出结论，镁橄榄石的晶体各向异性形状可以从红外光谱的峰特征推断出来。我们还表明，镁橄榄石蒸发和冷凝形成的晶体各向异性形状可以相互区分，并且可以从峰特征评估蒸发的温度条件。我们将目前的结果应用于原行星盘 HD100546 的红外光谱，发现一定比例（~ 25%）的镁橄榄石尘埃可能经历了高温蒸发（> 1600 K）。
Crystalline dust has been observed by infrared spectroscopy around dust-enshrouded asymptotic giant branch stars, in protoplanetary disks, and from some comets. Crystalline materials often have a specific shape related to a specific crystallographic orientation (crystallographically anisotropic shape), which reflects the anisotropic nature of crystals, and their infrared spectral features depend on crystallographically anisotropic shapes. The crystallographically anisotropic shape is thus a potentially powerful probe to evaluate circumstellar dust-forming conditions quantitatively. In order to assess the possibility to determine the crystallographically anisotropic shape from infrared spectra, we calculated mass absorption coefficients for ellipsoidal forsterite particles, the most abundant circumstellar crystalline silicate, elongated and flattened along the crystallographic a-, b-, and c-axes with various aspect ratios in the wavelength range of 9–70 μm. It was found that differences in infrared features caused by various crystallographicaly anisotropic shapes are distinguishable from each other irrespective of the effects of temperature, size, chemical composition, and grain edges of forsterite in the range of 9–12 μm and 15–20 μm. We thus concluded that the crystallographically anisotropic shape of forsterite can be deduced from peak features in infrared spectra. We also showed that the crystallographically anisotropic shapes formed by evaporation and condensation of forsterite can be distinguished from each other and the temperature condition for evaporation can be evaluated from the peak features. We applied the present results to the infrared spectrum of a protoplanetary disk HD100546 and found that a certain fraction (∼ 25%) of forsterite dust may have experienced high-temperature evaporation (> 1600 K).