[论文解读] The adventure of carbon stars - Observations and modelling of a set of C-rich AGB stars

Gioia Rau, Josef Hron+6 more|arXiv (Cornell University)|Jan 16, 2017

Stellar, planetary, and galactic studies参考文献 2被引用 15

一句话总结

本研究结合VLTI/MIDI干涉测量、光谱测光以及动力模型大气，研究了六颗富碳渐近巨星支（AGB）星的大气结构与质量损失。模型成功再现了红移方向的SED和可见度数据，揭示出Mira型星具有延伸的壳层状结构并经历周期性质量损失，而非Mira型星则更紧凑且质量损失率较低，与热脉动AGB（TP-AGB）演化模型一致。

ABSTRACT

Modeling stellar atmospheres is a complex, intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain them. Aims: We performed a self-consistent modeling of the atmospheres of six C-rich AGB stars: R Lep, R Vol, Y Pav, AQ Sgr, U Hya and X TrA, with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres. Methods. We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent dynamic models atmospheres. Results: The models can reproduce SED data well at wavelengths longwards of 1μm, and the interferometric observations between 8μm and 10μm. Discrepancies could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, with a prominent shell-like structure, while the models best fitting the non-Miras are more compact, showing lower average mass-loss. The mass loss is of episodic or multi-periodic nature, but causes the visual amplitudes to be notably larger than the observed ones. Stellar parameters were derived from the model fitting: T_eff, L_bol, M, C/O, $\dot{ ext{M}}$. Our findings agree well with literature values within the uncertainties. T_eff and L_bol are also in good agreement with the T derived from the angular diameter θ_{(V-K)} and L_bol from the SED fitting, except for AQ Sgr. Finally, θ_Ross and θ_(V-K) agree with each other better for the Miras than for the non-Miras,which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary tracks. We found that the main derived properties from the model fitting are in good agreement with TP-AGB evolutionary calculations for carbon stars (COLIBRI code).

研究动机与目标

• 通过多技术观测与自洽建模，理解富碳AGB星中的动态大气过程。
• 通过模型拟合约束有效温度、光度、质量、C/O比和质量损失率等恒星参数。
• 研究脉动与尘埃驱动风在塑造富碳AGB星周围环境中的作用。
• 检验当前动态模型大气（DARWIN）在再现观测SED与可见度曲线方面的局限性，特别是在可见光与中红外波段。
• 将模型预测与干涉测量数据对比，评估尘埃颗粒尺寸与消光系数假设对模型精度的影响。

提出的方法

• 利用VLTI/MIDI高角分辨率干涉测量数据，探测包层的空间结构。
• 将干涉测量数据与光学至中红外波段的光谱测光数据相结合。
• 应用自洽的动力模型大气（DARWIN），求解包含尘埃形成与增长的时间依赖流体动力学与辐射转移方程。
• 通过变化恒星参数与尘埃性质的模型网格，拟合观测到的SED与可见度曲线。
• 通过比较包含与不包含小颗粒极限（SPL）假设的模型，检验其对尘埃消光系数与模型拟合质量的影响。
• 利用几何模型与角直径测量（θ(V-K) 与 θRoss）交叉验证有效温度与总光度。

实验结果

研究问题

• RQ1 动态模型大气在多大程度上能再现富碳AGB星的观测SED与可见度曲线？
• RQ2 与Mira型星相比，非Mira型星中周期性或多重周期脉动在塑造质量损失与大气结构方面起到何种作用？
• RQ3 当小颗粒极限被放宽时，尘埃颗粒尺寸依赖的消光系数在多大程度上影响模型对中红外干涉数据的拟合？
• RQ4 所推导的恒星参数（Teff, Lbol, M, C/O, Ṁ）与演化模型及文献值相比如何？
• RQ5 为何模型中的可见光振幅显著高于观测值，尤其在半规则变星与不规则变星中？

主要发现

• 模型在大于1 μm的波长范围内良好再现SED，在8至10 μm波段的可见度数据上也表现出与观测的良好一致性，表明在中红外波段与观测高度吻合。
• Mira型星表现出显著延伸的大气层，具有明显的壳层状结构，而非Mira型星则更紧凑，平均质量损失率较低。
• 模型与观测在可见光振幅上的差异可能源于非Mira型星中脉动的周期性或多重周期性特征，而这些特征在模型中尚未被充分捕捉。
• 模型推导的有效温度与总光度与基于角直径（θ(V-K)）和SED拟合的结果高度一致，仅AQ Sgr存在偏差，可能归因于相位相关的变异性或模型局限性。
• 未采用小颗粒极限（SPL）假设的模型表现出更低的尘埃凝结度，且能更好地解释部分恒星的可见光超额，提示未来模型需引入颗粒尺寸依赖的消光系数。
• 恒星在H-R图中的位置与使用COLIBRI代码计算的TP-AGB演化轨迹高度一致，证实与当前恒星演化理论的一致性。