[论文解读] Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss

T. Khouri, L. B. F. M. Waters+6 more|arXiv (Cornell University)|Mar 16, 2015

Stellar, planetary, and galactic studies参考文献 55被引用 19

一句话总结

本研究利用多波段观测与辐射转移模型，对富氧S型渐近巨星支星W Hydrae的尘埃风进行建模，以约束尘埃成分、结构及质量损失历史。研究识别出一个以非晶态Al2O3为主、受引力束缚的尘埃壳层，解释了红外过量与散射光；硅酸盐发射源于40 AU以外，归因于近红外波段光学厚度增强；并揭示约3,500年前发生过一次近期的质量损失峰值，持续约300年。

ABSTRACT

Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives in a phase known as the asymptotic giant branch (AGB). During the AGB a significant fraction of their initial mass is expelled in a stellar wind. This process controls the final stages of their evolution and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. We modelled the dust envelope of W Hya using an advanced radiative transfer code. The dust model was analysed in the light of a previously calculated gas-phase wind model and compared to measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al$_2$O$_3$ emission. The composition of the large ($\sim$\,0.3\,$μ$m) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star and we find that they need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. Finally, the recent mass loss of W Hya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years.

研究动机与目标

• 理解W Hydrae（一颗富氧AGB星）风中尘埃成分与结构。
• 利用多波段观测约束W Hya的质量损失率历史。
• 研究Al2O3和硅酸盐等尘埃物种在驱动恒星风中的作用。
• 将观测到的红外光谱、散射光比例与尘埃图像与物理解释一致的尘埃模型相协调。
• 评估含钙尘埃物种及其消光性质在风中的贡献。

提出的方法

• 采用先进的辐射转移代码，对W Hya的尘埃包层在多个波段进行建模。
• 结合ISO红外光谱、PACS 70 µm图像、MIDI干涉数据及散射光比例的约束。
• 使用受引力束缚尘埃壳层（GBDS）模型，并采用修正的Rice-Nordheim（MRN）尺寸分布描述非晶态Al2O3颗粒。
• 整合先前研究（论文I）中的气相风模型，以确保与观测到的CO谱线轮廓及质量损失率一致。
• 探索尘埃消光系数的变化，特别是硅酸盐在近红外波段的消光增强，以解释大半径处的发射。
• 利用现有光学常数测试含钙尘埃物种，但受限于数据不足，难以实现精确量化。

实验结果

研究问题

• RQ1 是什么主导尘埃物种导致了W Hya风中观测到的红外过量与散射光？
• RQ2 尘埃发射结构如何与从CO谱线推断出的运动学及质量损失历史相关联？
• RQ3 大距离处（40 AU以外）观测到的硅酸盐发射能否通过尘埃消光特性与颗粒增长得到解释？
• RQ4 钙在尘埃成分中扮演什么角色？其丰度能否通过观测发射得到约束？
• RQ5 W Hya的质量损失率的时间演化如何？最近一次峰值发生在何时？

主要发现

• W Hya的尘埃光谱最合理地由一个以非晶态Al2O3为主、受引力束缚的尘埃壳层（GBDS）解释，该壳层可说明观测到的红外过量与散射光比例。
• 热红外波段的硅酸盐发射起源于40 AU以外，需显著增强的近红外波段光学厚度才能在如此大距离上可见。
• 硅酸盐在大半径处近红外波段消光系数的增加，与从CO谱线轮廓推断出的膨胀速度突然上升相吻合。
• 模型表明钙可能为尘埃的组成部分，但含钙物种的光学常数数据有限，导致其丰度难以精确量化。
• W Hya近期的质量损失历史显示约3,500年前出现一次峰值，持续约300年，质量损失率最高可达当前水平的20倍。
• 此前一个质量损失率较低的阶段（较当前水平低约两倍）持续了约2,500年，形成了如今位于500至4,000 AU之间的尘埃。