[论文解读] Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars: C_{60} fullerenes and polycyclic aromatic hydrocarbons

C. Gielen, J. Cami+3 more|arXiv (Cornell University)|Oct 27, 2011

Astrophysics and Star Formation Studies参考文献 69被引用 68

一句话总结

本研究首次在双星后渐近巨支（post-AGB）星的富氧周恒星环境中探测到C60富勒烯和多环芳烃（PAHs），挑战了传统观点，即此类碳质分子仅在富碳喷流中形成。利用斯皮策空间望远镜IRS光谱，作者发现尽管中心恒星及其周双星盘具有富氧特性，大型碳质分子仍能持续存在，表明可能存在非平衡过程或冲击驱动的PAHs向富勒烯转化等意外化学过程。

ABSTRACT

Context. The circumstellar environment of evolved stars is generally rich in molecular gas and dust. Typically, the entire environment is either oxygen-rich or carbon-rich, depending on the evolution of the central star. Aims. In this paper we discuss three evolved disc sources with evidence of atypical emission lines in their infrared spectra. The stars were taken from a larger sample of post-AGB binaries for which we have Spitzer infrared spectra, characterised by the presence of a stable oxygen-rich circumbinary disc. Our previous studies have shown that the infrared spectra of post-AGB disc sources are dominated by silicate dust emission, often with an extremely high crystallinity fraction. However, the three sources described here are selected because they show a peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we study in detail the peculiar mineralogy of the three sample stars. Using the observed emission features, we identify the different observed dust, molecular and gas species. Results. The infrared spectra show emission features due to various oxygen-rich dust components, as well as CO2 gas. All three sources show the strong infrared bands generally ascribed to polycyclic aromatic hydrocarbons. Furthermore, two sample sources show C60 fullerene bands. Conclusions. Even though the majority of post-AGB disc sources are dominated by silicate dust in their circumstellar environment, we do find evidence that, for some sources at least, additional processing must occur to explain the presence of large carbonaceous molecules. There is evidence that some of these sources are still oxygen-rich, which makes the detection of these molecules even more surprising.

研究动机与目标

• 研究后渐近巨支双星富氧周恒星环境中碳质分子的存在情况，这些环境通常预期以硅酸盐尘埃为主。
• 确定在这些富氧系统中探测到PAHs和C60富勒烯是否表明存在非平衡化学或异常物理条件。
• 探讨在碳应被锁定为CO的环境中，大型碳质分子如何形成或存活的机制。
• 评估冲击、恒星脉动或催化过程在富氧盘中促成复杂碳质分子形成的作用。

提出的方法

• 分析三颗已知具有富氧周恒星盘的后渐近巨支双星的高分辨率和低分辨率斯皮策红外光谱仪（IRS）光谱。
• 通过5–20 µm波段内的特征发射谱线识别尘埃、气体和分子种类。
• 通过光谱拟合分离并量化非晶态和结晶态硅酸盐、CO2气体、PAHs和C60富勒烯的发射。
• 基于C60在7.8、8.5、11.2和17.1 µm处发射谱带的相对强度，估算其激发温度和质量。
• 将观测光谱与PAH和富勒烯发射的实验室及理论模型进行比较。
• 利用LIR/L∗比值作为盘质量与演化状态的诊断工具，探索其与分子成分的相关性。

实验结果

研究问题

• RQ1 如何在被归类为富氧的周恒星环境中探测到C60富勒烯和PAHs？
• RQ2 何种物理或化学过程可能使大型碳质分子在富氧盘中形成或存活？
• RQ3 CO2气体的存在是否与这些富氧系统中碳质分子的形成有关？
• RQ4 为何仅少数后渐近巨支盘源（约70个中的6个）被发现含有可探测的PAHs或富勒烯？

主要发现

• 所研究的三颗后渐近巨支双星——IRAS 06338、HD 52961和EP Lyr——表现出强烈的富氧硅酸盐、CO2气体、PAHs和C60富勒烯的发射特征。
• 本研究首次明确探测到富氧后渐近巨支系统中的C60富勒烯，其表现出特征性的7.8、8.5、11.2和17.1 µm发射谱带。
• HD 52961和IRAS 06338中C60的激发温度估计约为1500 K，表明其来自高度激发态的发射。
• 这些源中C60的质量量级为10−6至10−5 M⊙，表明富勒烯存在显著的生成或积累。
• 尽管被归类为富氧（C/O < 1），但PAHs和C60的存在表明碳并未完全被锁定在CO中，暗示存在活跃的化学加工过程。
• 这些源的LIR/L∗比值较低（3–12%），表明尘埃质量较小或盘更透明，可能增强气体谱线的可见性并影响光致离解速率。