[论文解读] MgS in detached shells around carbon stars. Mining the mass-loss history

S. Hony, J. Bouwman|ArXiv.org|Sep 29, 2003

Astrophysics and Star Formation Studies参考文献 29被引用 10

一句话总结

本研究通过辐射转移模型模拟了碳星周围孤立尘埃壳层中的MgS尘埃，结果表明，壳层中的MgS颗粒可解释ISO/SWS光谱中观测到的低温MgS发射特征。模型证实MgS位于壳层半径处，证明壳层形成于富碳演化阶段，且MgS特征的轮廓可用于通过颗粒温度估算距离。

ABSTRACT

We investigate the dust composition of detached shells around carbon stars, with a focus to understand the origin of the cool magnesium-sulfide (MgS) material around warm carbon stars, which has been detected around several of these objects. We build a radiative transfer model of a carbon star surrounded by an expanding detached shell of dust. The shell contains amorphous carbon grains and MgS grains. We find that a small fraction of MgS dust (2% of the dust mass) can give a significant contribution to the IRAS 25 micron flux. However, the presence of MgS in the detached shell cannot be inferred from the IRAS broadband photometry alone but requires infrared spectroscopy. We apply the model to the detached-shell sources R Scl and U Cam, both exhibiting a cool MgS feature in their ISO/SWS spectra. We use the shell parameters derived for the molecular shell, using the CO submillimetre maps. The models, with MgS grains located in the detached shell, explain the MgS grain temperature, as derived from their ISO spectra, very well. This demonstrates that the MgS grains are located at the distance of the detached shell, which is a direct indication that these shells originate from a time when the stellar photosphere was already carbon-rich. In the case of R Scl, the IRAS photometry is simultaneously explained by the single shell model. In the case of U Cam, the IRAS photometry is under predicted, pointing to a contribution from cooler dust located even further away from the star than the molecular shell. We present a simple diagnostic to constrain the distance of the shell using the profile of the MgS emission feature. The emission feature shifts to longer wavelength with decreasing grain temperature. One can therefore infer a temperature and a corresponding distance to the star from the observed profile. (abridged)

研究动机与目标

• 研究碳星周围孤立壳层中低温MgS尘埃的起源与位置。
• 确定MgS颗粒是否与分子壳层共位置，以推断壳层喷射时恒星的化学状态。
• 利用MgS发射特征轮廓开发一种诊断工具，通过颗粒温度估算壳层距离。
• 评估IRAS宽带测光在检测MgS方面的局限性，并判断红外光谱的必要性。
• 将模型应用于R Scl和U Cam等特定恒星，其ISO/SWS数据质量高且分子壳层已解析。

提出的方法

• 构建包含非晶态碳和MgS颗粒的碳星及其孤立尘埃壳层的辐射转移模型。
• 使用来自CO亚毫米波图（Lindqvist et al., 1999; Schöier & Olofsson, 2001）的壳层参数作为模型输入。
• 将模型生成的合成光谱与R Scl和U Cam的观测ISO/SWS光谱及IRAS测光进行比较。
• 应用MgS发射特征轮廓对颗粒温度的依赖关系，推断颗粒温度，进而估算壳层距离。
• 利用总光度和角半径估算其他MgS特征较弱的恒星的壳层半径。
• 通过变化MgS质量分数和壳层半径的模型网格，评估光谱特征对尘埃成分和几何结构的敏感性。

实验结果

研究问题

• RQ1 在具有孤立壳层的碳星中，MgS颗粒是否与分子壳层位于相同距离？
• RQ2 MgS发射特征轮廓是否可用于推断尘埃壳层的温度与距离？
• RQ3 MgS存在于孤立壳层中是否意味着恒星在壳层喷射时处于富碳状态？
• RQ4 尽管恒星本身光学明亮且温度高，为何MgS特征仍表现为低温？
• RQ5 IRAS宽带测光在多大程度上可检测到MgS，还是必须依赖光谱学？

主要发现

• 仅占尘埃总质量2%的MgS尘埃即可显著贡献于IRAS 25 μm通量，但仅靠宽带测光无法确认其存在。
• R Scl和U Cam的MgS发射特征最合理的解释是MgS颗粒位于孤立分子壳层的距离处，证实了共位置关系。
• R Scl的壳层与同时匹配IRAS测光和ISO/SWS光谱的单壳层模型一致。
• U Cam的壳层在IRAS测光中存在低估现象，表明分子壳层之外存在额外的较冷尘埃。
• 随着颗粒温度降低，MgS特征轮廓向长波方向移动，从而可作为诊断工具，通过观测轮廓估算壳层距离。
• MgS特征的温度依赖性可消除SED解释中的模糊性，并为未来利用SIRTF和SOFIA等仪器实现距离约束提供可能。