S型恒星W Aql令人惊讶的富碳环境
The surprisingly carbon-rich environment of the S-type star W Aql
摘要：
W Aql是一颗渐近巨星分支（AGB）星，大气元素丰度比C/O $\approx$ 0.98，星周分子丰度介于M型（C/O $<$ 1）和C型（C/O $>$ 1）AGB星之间。这种中间状态被认为是典型的S-型恒星，虽然我们的理解，其拱星包层（CSE）的化学成分目前相当有限。我们在159-268 GHz的频率范围内进行了观测与APEX望远镜，并通过比较现有的光谱对一些研究良好的AGB星和旋转图分析的基础上的情况下，SiC 2的丰度估计。我们的结论是，W Aql的CSE似乎相当接近的C型AGB星比M型AGB星。特别是，我们检测到C2 H，SiC 2，SiN和HC 3 N的发射，这些分子以前只检测到C型恒星的CSE。这一结论，基于化学的气体成分的CSE，进一步支持在文献中的报告存在的大气分子带和光谱特征的尘埃物种典型的C型AGB星。虽然我们的观测主要是在CSE的外部区域跟踪物种，我们的结论与最近的S型恒星内部风的化学平衡模型密切相关：S型恒星的大气和星周化学可能更接近于C型AGB恒星，而不是M型AGB恒星。需要对S型星的星周气体化学进行进一步的观测研究，以确定其对大气C/O的依赖性。AGB星的CSE的非平衡化学模型需要解决特定类型的S型星和由大气C/O的范围引起的化学变化。
英文摘要：
W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O$\approx$0.98 and reported circumstellar molecular abundances intermediate between those of M-type (C/O$<$1) and C-type (C/O$>$1) AGB stars. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes (CSEs) is currently rather limited. We performed observations in the frequency range 159-268 GHz with the APEX telescope and make abundance estimates through comparison to available spectra towards some well-studied AGB stars and based on rotational diagram analysis in the case of SiC2. We conclude that W Aql's CSE appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the CSEs of C-type stars. This conclusion, based on the chemistry of the gaseous component of the CSE, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the CSE, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the CSEs of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.
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