Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus: Non-LTE abundance analysis of CO, SiO, and HCN
Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus: Non-LTE abundance analysis of CO, SiO, and HCN
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富氧 AGB 星 R Doradus 星周包膜的化学含量：CO、SiO 和 HCN 的非 LTE 丰度分析
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发表时间：
2017
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通讯作者：
W. Homan
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作者：
M. Sande;L. Decin;R. Lombaert;T. Khouri;A. D. Koter;A. D. Koter;F. Wyrowski;R. D. Nutte;W. Homan
上下文低质量到中等质量恒星的恒星外流以丰富的化学物质为特征。分子气体物质凝结成尘埃颗粒是导致恒星风爆发的一系列物理过程中的关键组成部分。为了提高我们对微观尺度化学和宏观尺度动力学之间耦合的理解，我们需要检索整个流出物中分子的丰度。目标。我们的目标是确定SiO和HCN的径向丰度分布的整个恒星流出的R多尔，富氧AGB星星具有低的质量损失率。SiO被认为在富氧AGB星的尘埃形成过程中起着至关重要的作用。HCN在富氧环境中的存在被认为是由于内风中的非平衡化学。方法.我们分析了CO，SiO和HCN的分子跃迁测量的APEX望远镜和所有三个仪器上的赫歇尔空间天文台，在文献中提供的数据。测光数据和红外光谱测量的ISO-SWS被用来约束流出的灰尘成分。采用连续辐射传输法和线辐射传输法，建立了气体和粉尘的物理包络模型。我们进行了分析的SiO和HCN的分子跃迁，以计算其丰度。结果我们已经得到了一个信封模型，描述的尘埃和气体的流出，并确定了丰富的SiO和HCN整个区域的恒星流出我们的分子数据探测。对于SiO，我们发现其初始丰度在5.5 × 10 - 5 ~ 6.0 × 10 - 5之间.丰度曲线在60 ± 10 R范围内保持恒定，之后呈高斯曲线下降，e折叠半径为3.5 ± 0.5 × 1013 cm或1.4 ± 0.2R。对于HCN，我们发现相对于H2的初始丰度为5.0 × 10 - 7.描述这种下降的高斯分布从恒星表面开始，其e折叠半径re = 1.85 ± 0.05 × 1015 cm或74 ± 2 R.结论.我们不能明确地确定SiO在60 ± 10 R下被破坏的机理。发现的初始丰度高于以前确定的（除了以前的一项研究SiO），这可能是由于包含更高的J过渡。与高质量损失率的米拉星星IK Tau相比，SiO和HCN丰度的差异可能是由于中央星星的不同脉动特性和/或尘埃凝结物理学的差异。
Context. The stellar outflows of low- to intermediate-mass stars are characterised by a rich chemistry. Condensation of molecular gas species into dust grains is a key component in a chain of physical processes that leads to the onset of a stellar wind. In order to improve our understanding of the coupling between the micro-scale chemistry and macro-scale dynamics, we need to retrieve the abundance of molecules throughout the outflow. Aims. Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. SiO is thought to play an essential role in the dust-formation process of oxygen-rich AGB stars. The presence of HCN in an oxygen-rich environment is thought to be due to non-equilibrium chemistry in the inner wind. Methods. We analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with data available in the literature. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. Results. We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the stellar outflow probed by our molecular data. For SiO, we find that the initial abundance lies between 5.5 × 10 -5 and 6.0 × 10 -5 with respect to H 2 . The abundance profile is constant up to 60 ± 10 R, after which it declines following a Gaussian profile with an e-folding radius of 3.5 ± 0.5 × 10 13 cm or 1.4 ± 0.2 R. For HCN, we find an initial abundance of 5.0 × 10 -7 with respect to H 2 . The Gaussian profile that describes the decline starts at the stellar surface and has an e-folding radius r e of 1.85 ± 0.05 × 10 15 cm or 74 ± 2 R. Conclusions. We cannot unambiguously identify the mechanism by which SiO is destroyed at 60 ± 10 R. The initial abundances found are higher than previously determined (except for one previous study on SiO), which might be due to the inclusion of higher-J transitions. The difference in abundance for SiO and HCN compared to high mass-loss rate Mira star IK Tau might be due to different pulsation characteristics of the central star and/or a difference in dust condensation physics.