[论文解读] Synthetic photometry for carbon-rich giants. IV. An extensive grid of dynamic atmosphere and wind models

Kimmo Eriksson, W. Nowotny+3 more|arXiv (Cornell University)|Apr 29, 2014

Stellar, planetary, and galactic studies参考文献 38被引用 44

一句话总结

本文为太阳金属丰度的碳富集渐近巨星支（AGB）星提供了540个动态大气与风模型的网格，计算了低分辨率光谱与测光（0.35–25 μm），以研究质量损失与时变行为。模型在质量损失率与颜色方面与观测结果具有良好一致性，但风速的差异表明需改进尘埃消光系数假设与参数范围，而无需显著改变恒星演化模型中的质量损失率。

ABSTRACT

The evolution and spectral properties of stars on the AGB are significantly affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind models are an essential tool for studying these evolved stars, both individually and as members of stellar populations, to understand their contribution to the integrated light and chemical evolution of galaxies. This paper is part of a series testing state-of-the-art atmosphere and wind models of carbon stars against observations, and making them available for use in various theoretical and observational studies. We have computed low-resolution spectra and photometry (in the wavelength range 0.35-25 mu) for a grid of 540 dynamic models with stellar parameters typical of solar-metallicity C-rich AGB stars and with a range of pulsation amplitudes. The models cover the dynamic atmosphere and dusty outflow (if present), assuming spherical symmetry, and taking opacities of gas-phase species and dust grains consistently into account. To characterize the time-dependent dynamic and photometric behaviour of the models in a concise way we defined a number of classes for models with and without winds. Comparisons with observed data in general show a quite good agreement for example regarding mass-loss rates vs. (J-K) colours or K magnitudes vs. (J-K) colours. Some exceptions from the good overall agreement, however, are found and attributed to the range of input parameters (e.g. relatively high carbon excesses) or intrinsic model assumptions (e.g. small particle limit for grain opacities). While current results indicate that some changes in model assumptions and parameter ranges should be made in the future to bring certain synthetic observables into better agreement with observations, it seems unlikely that these pending improvements will significantly affect the mass-loss rates of the models.

研究动机与目标

• 开发一套一致的、公开可获取的碳富集AGB星动态大气与风模型网格，以支持理论与观测研究。
• 测试先进模型在碳富集巨星观测测光与光谱特性上的表现。
• 识别合成观测与实际观测之间的差异，以指导未来模型改进。
• 评估从模型中推导的质量损失率在恒星演化计算中的稳健性。
• 评估模型假设（尤其是尘埃消光系数极限与参数范围）对合成观测的影响。

提出的方法

• 针对540个具有球对称性与时变激波的动态模型，计算0.35–25 μm波段的低分辨率光谱与测光。
• 整合气体成分与尘埃颗粒的一致消光系数，包括小颗粒极限下的非晶态碳（amC）。
• 基于Mattsson等人（2010）的网格，采用典型太阳金属丰度碳富集AGB星的恒星参数，并变化脉动振幅。
• 根据动态行为与风的存在性定义模型类别，以紧凑表征时变测光与大气特性。
• 将合成观测（如(J–K)色指数、K星等，质量损失率）与2MASS与IRAS等巡天的观测数据进行比较。
• 评估结果对输入参数（如碳过量、光度、脉动振幅）的敏感性。

实验结果

研究问题

• RQ1 动态大气与风模型在多大程度上能准确再现碳富集AGB星的观测测光色指数（如(J–K)）与星等？
• RQ2 脉动振幅与碳过量的变化对合成质量损失率与风速有何影响？
• RQ3 模型假设（尤其是尘埃消光系数的小颗粒极限）在多大程度上影响合成观测？
• RQ4 当前模型对风速的预测是否与观测一致，特别是对低质量与低光度恒星？
• RQ5 尽管风速预测存在差异，这些模型推导出的质量损失率是否仍可可靠地用于恒星演化代码？

主要发现

• 模型在质量损失率与(J–K)色指数关系、K星等与(J–K)色指数关系方面与观测结果总体一致。
• 高碳过量的模型产生的风速过高，与观测恒星不符，提示此类恒星可能稀少，或需调整模型假设。
• 低(C–O)值与高质质量损失率的模型表现出不切实际的低风速，表明需引入尺寸依赖的尘埃消光系数以纠正此偏差。
• 初步研究表明，若使用尺寸依赖的尘埃消光系数替代小颗粒极限，可提高慢速风的风速，而质量损失率基本不变。
• 尽管风速预测存在差异，质量损失率仍具稳健性，未来模型改进不太可能显著改变其数值。
• Wachter等人（2002）提出的质量损失公式得出的质量损失率系统性偏高，鉴于与观测更一致，建议据此公式向下修正。