[论文解读] Cool luminous stars: the hybrid nature of their infrared spectra -- Carbon, oxygen, and their isotopic abundances in 23 K - M giant stars

Takashi Tsuji|arXiv (Cornell University)|Jul 28, 2008

Stellar, planetary, and galactic studies参考文献 1被引用 31

一句话总结

本研究揭示了K-型超巨星红外光谱的混合特性，表明只有弱线（log W/ν ≤ −4.75）在光球层丰度分析中可靠，因为强线受到温暖分子包层（MOLsphere）的污染。强线和中等强度线严重受此外部组分干扰，导致经典丰度分析方法失效，并揭示MOLsphere是冷亮星大气中的关键特征。

ABSTRACT

We identified a possible origin of the difficulty in abundance analysis of cool luminous stars. We found purely empirically that there is a limit of logW/nu = -4.75 (W: equivalent width, nu: wavenumber) above which the observed lines do not follow the line formation theory based on the classical micro-turbulent model and that the abundance analysis can be done only with the lines of logW/nu < -4.75. The C, O, and their isotopic abundances determined from such weak lines of CO and OH in 23 K - M giants are roughly consistent with the predictions of evolutionary models. However, the stronger lines of logW/nu > -4.75 cannot be analyzed at all by the classical line formation theory. From the behavior of these lines and considering other observations such as the detections of H2O lines, not only in the late M giants but also in the early M and K giants, we found that these lines are badly disturbed by the contamination from extra molecular layers. We already know that the very strong lines of logW/nu > -4.4 are contaminated by the contribution from the extra warm molecular layers, but we now show that such contamination should be prevailing not only in the strong low excitation lines but also in the intermediate-strength lines (-4.75 < logW/nu > -4.4) as well. The reason why these lines cannot be used for determining photospheric abundances is simply because they include the contamination of the non-photospheric origin. Instead they can be new proves of the warm molecular envelope for which little is known yet. An important conclusion is that the infrared spectra of K - M giants are a hybrid of at least two components originating in the photosphere and the warm molecular envelope. In the interpretation and analysis of the infrared spectra of cool luminous stars, it is essential to keep their hybrid naure in mind.

研究动机与目标

• 解决利用红外光谱分析冷亮星丰度时长期存在的困难。
• 确定K-型超巨星光谱线异常的成因，特别是经典线形成模型失效的原因。
• 利用高分辨率FTS数据测定23颗K-型超巨星的碳、氧及其同位素丰度。
• 评估MOLsphere（温暖分子包层）对光谱线的扭曲作用及其对丰度测定的影响。
• 确立一个临界阈值（log W/ν ≈ −4.75），用以区分可靠的弱线与受污染的中等及强线。

提出的方法

• 采用逐线分析法，结合经典微湍流模型和一维模型光球层。
• 使用基特峰国家天文台提供的高分辨率傅里叶变换光谱（FTS）数据，数据来源为Hinkle和Ridgway。
• 根据log W/ν将光谱线分为三类：弱线（≤ −4.75）、中等强度线（−4.75 < log W/ν ≤ −4.4）和强线（> −4.4）。
• 比较各组的线轮廓和等效宽度，以检测与经典理论的偏离。
• 结合观测证据（如H₂O线的检测）以支持外部分子层的存在。
• 评估推导出的丰度与恒星演化模型（特别是首次和第二次混杂过程）的一致性。

实验结果

研究问题

• RQ1 为何经典丰度分析方法在K-型超巨星的强线和中等强度线中失效？
• RQ2 何种物理机制导致中等强度线偏离经典线形成理论？
• RQ3 log W/ν的临界值在何处发生从光球层行为到受污染行为的转变？
• RQ4 MOLsphere及外部分子层在多大程度上影响同位素丰度测定的可靠性？
• RQ5 尽管强线存在污染，弱线（log W/ν ≤ −4.75）是否仍能提供一致且准确的光球层丰度？

主要发现

• 在log W/ν ≈ −4.75处存在一个临界阈值，用以区分符合经典线形成理论的线与不符合的线。
• 仅当log W/ν ≤ −4.75时，弱线才适用于光球层丰度分析，因其受MOLsphere影响最小。
• 中等强度线（−4.75 < log W/ν ≤ −4.4）表现出与强线无法区分的行为，且受MOLsphere污染，即使在高达2 eV的激发能下亦然。
• 强线（log W/ν > −4.4）已知受外部分子层污染，本研究证实该污染也延伸至中等强度线。
• 从弱线推导出的碳、氧及同位素丰度与首次和第二次混杂演化模型总体一致，但¹²C、¹³C和¹⁴N的偏差提示可能存在间歇性混合事件。
• 红外光谱的混合特性——结合光球层与MOLsphere贡献——解释了长期存在的丰度分析困难，并凸显了高分辨率光谱学在分离光球信号中的必要性。