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arXiv 0801.1118astro-ph
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作为非平衡化学示踪剂的来自AGB星内部风的“母体”分子的探测

Detection of `parent' molecules from the inner wind of AGB stars as tracers of non-equilibrium chemistry

L. Decin, I. Cherchneff, S. Hony, S. Dehaes, C. De Breuck, K. M. Menten

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8 pages, 8 figures, accepted for publication in Astronomy & Astrophysics
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摘要：

渐近巨星分支（AGB）恒星的典型特征是强烈的尘埃驱动的分子外流。长期以来，人们认为AGB星的星周包层的分子结构主要由大气中的C/O比决定。然而，最近的观察分子，如HCN，SiO和SO揭示气相丰度高于预测的热力学平衡（TE）模型。紫外光子引发的外包层离解或内包层激波作用下的非平衡形成可能是异常丰度的来源。我们的目标是检测（i.）一组“母体”分子（CO、SiO、HCN、CS），根据Cherchneff（2006年）的非平衡研究预测，其形成丰度几乎恒定，与C/O比和渐近巨星分支（AGB）上的恒星演化阶段无关，以及（ii.）少数分子，如SiS和SO，对恒星富含O或C的性质敏感。用JCMT和APEX在毫米和亚毫米波段观测了四颗AGB星的几个关键分子的低激发和高激发转动跃迁：富氧的Mira WX Psc，S星W Aql，以及两颗碳星V Cyg和II Lup。进行临界密度分析，以确定高激发分子线的形成区域。我们在所有四个物体中检测到四个“母体”分子，这意味着，事实上，无论AGB的进化阶段如何，这些化学物质都是形成的。高激发线的SiS也检测到在三颗星与APEX，而SO只检测到富氧星WX Psc。这是第一个多分子观测证明，周期性冲击层以上的光球AGB星显示出一定的化学均匀性，无论光球的C/O比和阶段的恒星的演变。

英文摘要：

Asymptotic Giant Branch (AGB) stars are typified by strong dust-driven, molecular outflows. For long, it was believed that the molecular setup of the circumstellar envelope of AGB stars is primarily determined by the atmospheric C/O ratio. However, recent observations of molecules such as HCN, SiO, and SO reveal gas-phase abundances higher than predicted by thermodynamic equilibrium (TE) models. UV-photon initiated dissociation in the outer envelope or non-equilibrium formation by the effect of shocks in the inner envelope may be the origin of the anomolous abundances. We aim at detecting (i.) a group of `parent' molecules (CO, SiO, HCN, CS), predicted by the non-equilibrium study of Cherchneff (2006) to form with almost constant abundances independent of the C/O ratio and the stellar evolutionary stage on the Asymptotic Giant Branch (AGB), and (ii.) few molecules, such as SiS and SO,which are sensitive to the O- or C-rich nature of the star. Several low and high excitation rotational transitions of key molecules are observed at mm and sub-mm wavelengths with JCMT and APEX in four AGB stars: the oxygen-rich Mira WX Psc, the S star W Aql, and the two carbon stars V Cyg and II Lup. A critical density analysis is performed to determine the formation region of the high-excitation molecular lines. We detect the four `parent' molecules in all four objects, implying that, indeed, these chemical species form whatever the stage of evolution on the AGB. High-excitation lines of SiS are also detected in three stars with APEX, whereas SO is only detected in the oxygen-rich star WX Psc. This is the first multi-molecular observational proof that periodically shocked layers above the photosphere of AGB stars show some chemical homogeneity, whatever the photospheric C/O ratio and stage of evolution of the star.

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