[论文解读] Crystalline silicate dust around evolved stars II. The crystalline silicate complexes

F. J. Molster, L. B. F. M. Waters+1 more|UvA-DARE (University of Amsterdam)|Jan 18, 2002

Phase Equilibria and Thermodynamics参考文献 12被引用 123

一句话总结

该论文利用ISO红外光谱分析演化中的富氧星体中石英质硅酸盐的发射谱带，识别出49条窄带，分为七个光谱复合体（10–60 μm）。研究发现，富含镁的橄榄石和辉石占主导地位，且盘状结构（强谱带，高结晶度）与喷流结构（弱谱带，水冰主导）之间存在明显的光谱差异，表明其形成历史不同，分别源于高温与低温结晶过程。

ABSTRACT

This is the second paper in a series of three in which we present an exhaustive inventory of the 49 solid state emission bands observed in a sample of 17 oxygen-rich dust shells surrounding evolved stars. Most of these emission bands are concentrated in well defined spectral regions (called complexes). We define 7 of these complexes; the 10, 18, 23, 28, 33, 40 and 60 micron complex. We derive average properties of the individual bands. Comparison with laboratory data suggests that both olivines (Mg(2x)Fe(2-2x)SiO(4)) and pyroxenes (Mg(x)Fe(1-x)SiO(3)) are present, with x close to 1, i.e. the minerals are very Mg-rich and Fe-poor. This composition is similar to that seen in disks surrounding young stars and in the solar system comet Hale-Bopp. A significant fraction of the emission bands cannot be identified with either olivines or pyroxenes. Possible other materials that may be the carriers of these unidentified bands are briefly discussed. There is a natural division into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). In particular, stars with the 33.5 micron olivine band stronger than about 20 percent over continuum are invariably disk sources. Likewise, the 60 micron region is dominated by crystalline silicates in the disk sources, while it is dominated by crystalline H(2)O ice in the outflow sources. We show that the disk and outflow sources have significant differences in the shape of the emission bands. This difference must be related to the composition or grain shapes of the dust particles. The incredible richness of the crystalline silicate spectra observed by ISO allows detailed studies of the mineralogy of these dust shells, and is the origin and history of the dust.

研究动机与目标

• 利用高分辨率红外光谱识别并表征演化星体中的结晶硅酸盐发射谱带。
• 确定富氧包层环境中尘埃的矿物组成。
• 基于光谱特征，区分盘状与喷流几何结构中尘埃形成过程的差异。
• 评估结晶温度与颗粒结构对观测到的红外谱带形状的影响。
• 识别未归属的谱带，并评估对宇宙尘埃模拟物新实验室数据的需求。

提出的方法

• 分析覆盖2–200 μm波段的ISO短波与长波光谱仪数据。
• 通过光谱拟合并与实验室数据对比，识别窄发射谱带。
• 将谱带分类为七个光谱复合体：10、18、23、28、33、40和60 μm。
• 将观测到的谱带形状与强度与橄榄石（Mg₂xFe₂-2xSiO₄）和辉石（MgₓFe₁₋ₓSiO₃）的实验室光谱进行比较。
• 评估颗粒形貌的影响，包括顽辉石中晶体学轴向下陷，以解释与实验室数据的光谱偏差。
• 评估寒冷、致密团块环境中的H₂O冰结晶度与屏蔽机制。

实验结果

研究问题

• RQ1 演化中的富氧星体尘埃壳中，主导的结晶硅酸盐矿物是什么？
• RQ2 盘状与喷流几何结构中，结晶硅酸盐谱带的光谱特征有何不同？
• RQ3 10–60 μm发射谱带的相对强度与形状揭示了尘埃形成温度与环境的哪些信息？
• RQ4 为何部分观测谱带与橄榄石和辉石的标准实验室光谱不匹配？
• RQ5 在寒冷、受紫外辐射暴露的环境中，结晶H₂O冰通过何种机制得以存活？

主要发现

• 识别出49条窄发射谱带，分为10至60 μm之间的七个不同光谱复合体。
• 所有盘状源中，33.5 μm橄榄石谱带强度超过连续谱的25%，可作为盘状几何结构的明确形态指标。
• 盘状源表现出强烈的60 μm结晶硅酸盐发射，而喷流源在同一波段则以结晶H₂O冰为主导。
• 绝大多数谱带最合理地由富含镁的橄榄石和辉石（x ≈ 1）解释，其成分与原行星盘及海尔-波普彗星中的物质相似。
• 约20%的观测特征尚未归属，强度比值表明除橄榄石和辉石外还存在其他少量成分。
• 顽辉石中晶体学轴向下陷可能解释与实验室数据的光谱偏差，支持其在低密度喷流中通过气相成核的起源。