[论文解读] Line formation in AGB atmospheres including velocity effects. Molecular line profile variations of long period variables

W. Nowotny, S. Höfner+1 more|arXiv (Cornell University)|Feb 9, 2010

Stellar, planetary, and galactic studies参考文献 66被引用 52

一句话总结

本研究为AGB恒星开发了包含脉动引起的速度场和尘埃驱动风的动态模型大气，以模拟近红外光谱中的分子谱线轮廓。该模型成功再现了Mira变星和半规则变星的观测谱线轮廓变化与径向速度曲线，表明CO和CN谱线在大气中不同深度形成，且速度到径向速度转换因子约为1.2–1.5。

ABSTRACT

The atmospheres of evolved red giants are considerably influenced by pulsations of the stellar interiors and developing stellar winds. The resulting complex velocity fields severely affect molecular line profiles observable in NIR spectra. With the help of model calculations the complex line formation process in AGB atmospheres was explored with the focus on velocity effects. Furthermore, we aimed for atmospheric models which are able to quantitatively reproduce line profile variations found in observed spectra of pulsating late-type giants. Models describing pulsation-enhanced dust-driven winds were used to compute synthetic spectra under the assumptions of chemical equilibrium and LTE and by solving the radiative transfer in spherical geometry including velocity effects. Radial velocities derived from Doppler-shifted synthetic line profiles provide information on the gas velocities in the line-forming region of the spectral features. On the basis of dynamic models we investigated in detail the finding that various molecular features in AGB spectra originate at different geometrical depths of the very extended atmospheres. We show that the models are able to quantitatively reproduce the characteristic line profile variations of lines sampling the deep photosphere. The global velocity fields of typical LPVs are also realistically reproduced. Possible reasons for discrepancies concerning other modelling results are outlined. In addition, we present a model showing variations of CO dv=3 line profiles comparable to observed spectra of SRVs and discuss that the non-occurence of line doubling in these objects may be due to a density effect. The results of our line profile modelling are another indication that the dynamic models studied here are approaching a realistic representation of the outer layers of AGB stars.

研究动机与目标

• 理解径向脉动与尘埃驱动风如何塑造AGB恒星大气中分子谱线轮廓。
• 在扩展的大气层中建立复杂速度场的模型，并将其与高分辨率光谱中可观测的径向速度（RV）位移关联起来。
• 重现长周期变星中观测到的时间依赖谱线轮廓变化，特别是Mira变星和半规则变星。
• 量化谱线形成区域中真实气体速度与测量径向速度之间的关系。
• 评估现有模型在再现CO第一倍频跃迁线方面相对于第二倍频跃迁线的局限性。

提出的方法

• 在动态模型大气中使用分子谱线列表（CO和CN）进行球对称辐射转移计算。
• 应用包含脉动增强的尘埃驱动风的流体动力学模型，以模拟时间依赖的速度结构。
• 在模型中假设分子物种处于化学平衡和局部热动平衡（LTE）状态。
• 通过谱线轮廓中的多普勒位移计算合成光谱并推导径向速度，与观测结果进行比较。
• 利用径向光学深度分布确定大气中谱线的形成深度。
• 通过在静止初始模型上施加人工速度场测试速度到RV的转换关系，得到p ≈ 1.2–1.5。

实验结果

研究问题

• RQ1 脉动引起的速度场如何影响AGB恒星大气中分子谱线的形成与轮廓？
• RQ2 动态模型大气在多大程度上能够再现Mira变星和半规则变星中观测到的时间依赖谱线轮廓变化？
• RQ3 谱线轮廓中真实气体速度与测量径向速度之间的定量关系是什么？
• RQ4 为何某些CO谱线（如Δv = 2）与观测结果不符，尽管其他谱线（如Δv = 3）拟合良好？
• RQ5 AGB恒星中双峰谱线轮廓出现所需的大气条件是什么？

主要发现

• 在均匀速度场模型中，气体速度与测量径向速度之间的转换因子约为p = u_gas / RV ≈ 1.2–1.5。
• Mira变星中的CO Δv = 3和CN谱线在大气中不同深度形成，其径向速度曲线呈S形，与观测结果一致。
• 模型成功再现了Mira恒星的不连续、时间可变的谱线轮廓，包括特征性的RV曲线形态。
• CO Δv = 3的谱线双峰并非仅由激波引起；下落层的高光学深度也是使红移成分可见的必要条件。
• 半规则变星的模型表现出与观测相符的谱线轮廓变化，表明谱线双峰的缺失可能源于密度效应，而非缺乏激波。
• 尽管CO Δv = 3谱线拟合良好，但CO Δv = 2谱线与观测结果不符，表明在第一倍频跃迁区域建模仍具挑战，可能由于复杂动力学与尘埃消光效应的共同作用。