[论文解读] Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae

E. De Beck, L. Decin+5 more|UvA-DARE (University of Amsterdam)|Aug 5, 2010

Stellar, planetary, and galactic studies参考文献 88被引用 129

一句话总结

本文推导出用于估算演化后期恒星（包括渐近巨星分支星、红超巨星和黄超巨星）质量损失率的解析公式，基于八条跃迁的CO转动谱线轮廓。通过在包层中建模非局部热动平衡辐射转移，该方法考虑了谱线饱和与空间分辨率效应，从而实现对高质损率的精确估算，其中CO(2–1)提供最可靠的单条谱线估计。

ABSTRACT

We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of asymptotic giant branch (AGB), red supergiant (RSG), and yellow hypergiant stars in our galactic sample. Rotationally excited lines of CO are a very robust diagnostic in the study of circumstellar envelopes (CSEs). When sampling different layers of the CSE, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the CSEs of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule, apply them to our extensive CO data set covering 47 stars, and compare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the AGB stars range from 4x10^-8 Msun/yr up to 8x10^-5 Msun/yr. For RSGs they reach values between 2x10^-7 Msun/yr and 3x10^-4 Msun/yr. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the AGB stars and their derived mass-loss rates, with a levelling off at approx. 3x10^-5 Msun/yr for periods exceeding 850 days.

研究动机与目标

• 推导适用于利用CO转动谱线轮廓估算演化后期恒星质量损失率的一般性、解析表达式。
• 在推导质量损失率公式时，考虑谱线饱和与包层分辨效应，以实现高精度估算。
• 将这些公式应用于47颗演化后期恒星样本，估算其质量损失率并检测变异性。
• 研究AGB恒星脉动周期与质量损失率之间的关系。
• 从CO谱线强度比估算12C/13C同位素比值，作为恒星演化状态的探测手段。

提出的方法

• 在共动参考系中使用非局部热动平衡（non-LTE）辐射转移代码，模拟包层中CO谱线的强度。
• 推导恒星参数（如光度、温度）与CO谱线可观测量（如线宽、光学厚度、积分流量）之间的解析关系，以估算质量损失率。
• 将该理论框架应用于47颗在8条CO转动跃迁（J=1–0至J=8–7）中观测到的演化后期恒星，涵盖低激发与高激发谱线。
• 利用(V–K)0色指数和经验温度校准，校正星际与包层消光。
• 选择CO(2–1)谱线作为主要参考，因其在灵敏度与谱线饱和效应之间达到最佳平衡。
• 利用多条CO谱线数据检测质量损失率的时间变异性，并通过谱线强度比推导12C/13C比值。

实验结果

研究问题

• RQ1 哪些解析公式能可靠地估算演化后期恒星的质量损失率，同时考虑谱线饱和与空间分辨率效应？
• RQ2 哪条CO跃迁能为演化后期恒星提供最准确的质量损失率单条谱线估计？
• RQ3 AGB星与红超巨星的质量损失率如何与它们的脉动周期相关？
• RQ4 能否从多 epoch CO谱线数据中检测到质量损失率的时间变异性？
• RQ5 演化后期恒星的12C/13C同位素比值是多少？它如何反映其演化状态？

主要发现

• 推导出的解析公式可在8条CO转动跃迁中准确估算质量损失率，其中CO(2–1)提供最可靠的单条谱线估计。
• AGB恒星的质量损失率范围为4×10⁻⁸ M⊙ yr⁻¹至8×10⁻⁵ M⊙ yr⁻¹，而红超巨星的质量损失率介于2×10⁻⁷ M⊙ yr⁻¹与3×10⁻⁴ M⊙ yr⁻¹之间。
• 样本中7颗恒星显示出质量损失率时间变异性的证据，可通过多条CO轮廓分析检测。
• AGB恒星的脉动周期与质量损失率之间存在明显相关性，当周期超过850天时，质量损失率趋于平台，约为~3×10⁻⁵ M⊙ yr⁻¹。
• 12C/13C同位素比值通过CO谱线强度比推导，为恒星化学演化与演化状态提供探测手段。
• 该方法成功考虑了谱线饱和与空间分辨率效应，实现了对高达~10⁻⁴ M⊙ yr⁻¹质量损失率的稳健估算。