Sulphur-bearing molecules in AGB stars
Sulphur-bearing molecules in AGB stars
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DOI：
10.1051/0004-6361/201731203
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发表时间：
2017-10-24
影响因子：
6.5
通讯作者：
Millar, T. J.
中科院分区：
文献类型：
--
作者：
Danilovich, T.;Van de Sande, M.;Millar, T. J.
上下文。硫在本星系中是一种相对丰富的元素，已知在AGB恒星的星周包层中形成各种分子。这些分子的丰度因AGB恒星的化学类型和质量损失率而异。通过对各种含硫分子(亚)毫米发射线的测量，我们的目标是确定哪些分子是不同类型的AGB星中硫的主要载流子。本文是一系列研究中的第一篇，我们调查了AGB星周包层中H_2S的存在，并确定了它的丰度，如果可能的话。我们使用阿塔卡马探路者实验(APEX)望远镜观测了20颗具有不同质量损失率和不同化学类型的AGB恒星，以寻找五个关键含硫分子：CS、SIS、SO、SO_2和H_2S的旋转跃迁线。在这里，我们介绍了我们对硫化氢的结果，包括探测，非探测，以及对探测到的谱线进行详细的辐射传递模拟。我们比较了基于对硫化氢分子激发的各种描述和不同丰度分布的结果，如果可能的话，包括高斯丰度，以及从化学模拟结果得出的两种不同的丰度分布。我们对五颗AGB星探测到了H_2S，它们都有很高的质量损失率。M>；=5×10(-6)M圆点yr(-1)，富氧。对于落在类似质量损失范围内的碳星或S型恒星，没有检测到硫化氢。对于我们的探测样本中的恒星，我们发现相对于H-2的O-H_2S丰度峰值在4×10(-7)到2.5×10(-5)之间。总体而言，我们得出的结论是，在质量损失率较高的富氧AGB星中，H_2S可以发挥重要作用，但在其他化学类型的恒星或质量损失率较低的富氧星中，H_2S不太可能发挥关键作用。对于V1300Aql和Gx Mon这两个源，H_2S很可能是星周包层中的主要含硫分子。
Context. Sulphur is a relatively abundant element in the local Galaxy that is known to form a variety of molecules in the circumstellar envelopes of AGB stars. The abundances of these molecules vary based on the chemical types and mass-loss rates of AGB stars.Aims. Through a survey of (sub-) millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H2S in AGB circumstellar envelopes and determine its abundance, where possible.Methods. We surveyed 20 AGB stars with a range of mass-loss rates and different chemical types using the Atacama Pathfinder Experiment (APEX) telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO2, and H2S. Here we present our results for H2S, including detections, non-detections, and detailed radiative transfer modelling of the detected lines. We compared results based on various descriptions of the molecular excitation of H2S and different abundance distributions, including Gaussian abundances, where possible, and two different abundance distributions derived from chemical modelling results.Results. We detected H2S towards five AGB stars, all of which have high mass-loss rates of. M >= 5 x 10(-6) M-circle dot yr(-1) and are oxygen rich. H2S was not detected towards the carbon or S-type stars that fall in a similar mass-loss range. For the stars in our sample with detections, we find peak o-H2S abundances relative to H-2 between 4 x 10(-7) and 2.5 x 10(-5).Conclusions. Overall, we conclude that H2S can play a significant role in oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a key role in stars of other chemical types or in lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H2S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.