S星周围尘埃成分的ISO/SWS研究
An ISO/SWS study of the dust composition around S stars
摘要：
我们研究了S-型AGB星周围星风中固态物质的组成。S星在风中产生的尘埃与一些富含O的AGB星产生的尘埃相似。然而，报道的相似性主要是基于有限的光谱分辨率，灵敏度和波长覆盖范围的IRAS/LRS光谱。我们使用ISO/SWS数据研究了S星周围的尘埃成分，这些数据在光谱分辨率和波长覆盖范围方面超过了以前的研究。我们比较的尘埃光谱从9个来源的富O AGB光谱和一个子集的M超巨星。我们构建了不同类别的平均尘埃发射光谱。我们报告发现了一些以前未报道的尘埃排放功能的S星光谱。WAql和pi 1 Gru的长波长光谱显示出MgS的“30”微米特征。两个来源表现出一系列的发射带之间的20和40微米，我们暂时归因于透辉石。结果表明，S星的10-20 μ m光谱与富氧AGB星的光谱有很大的不同。富氧恒星呈现出结构化的发射特征，被认为是由无定形硅酸盐和氧化铝产生的。S星缺乏富O星的结构。相反，它们显示了一个平滑的峰值，从源到源具有不同的峰值位置。我们认为，这一功能是由一个家庭的相关材料，其确切的组成决定了峰的位置。观察到的趋势模拟了非化学计量硅酸盐的实验室趋势。在这种情况下，非化学计量的程度与Mg与SiO 4的比率有关，换句话说，与尘埃颗粒生长期间可用的游离O的量有关。
英文摘要：
We investigate the composition of the solid-state materials in the winds around S-type AGB stars. The S stars produce dust in their wind that bears a resemblance to the dust produced in some O-rich AGB stars. However, the reported resemblance is mostly based on IRAS/LRS spectra with limited spectral resolution, sensitivity, and wavelength coverage. We investigate the dust composition around S stars using ISO/SWS data that surpass the previous studies in terms of spectral resolution and wavelength coverage. We compare the dust spectra from the 9 sources with the O-rich AGB spectra and a subset of M super-giants. We constructed average dust emission spectra of the different categories. We report the discovery of several previously unreported dust emission features in the S star spectra. The long wavelength spectra of W Aql and pi1 Gru exhibit the "30" micrometer feature attributed to MgS. Two sources exhibit a series of emission bands between 20 and 40 micrometer that we tentatively ascribe to Diopside. We show that the 10-20 micrometer spectra of the S stars are significantly different from the O-rich AGB stars. The O-rich stars exhibit a structured emission feature that is believed to arise from amorphous silicate and aluminium-oxide. The S stars lack the substructure found in the O-rich stars. Instead they show a smooth peak with a varying peak-position from source to source. We suggest that this feature is caused by a family of related material, whose exact composition determines the peak position. The observed trend mimics the laboratory trend of non-stoichiometric silicates. In this scenario the degree of non-stoichiometry is related to the Mg to SiO4 ratio, in other words, to the amount of free O available during the dust grain growth.
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