Water isotopologues in the circumstellar envelopes of M-type AGB stars
Water isotopologues in the circumstellar envelopes of M-type AGB stars
复制标题
M型AGB星的星周包层中的水同位素体
DOI：
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
H. Olofsson
中科院分区：
文献类型：
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作者：
T. Danilovich;R. Lombaert;L. Decin;A. Karakas;A. Karakas;M. Maercker;H. Olofsson
目的：研究水的两种同位素H2 $$^{17}$O和H2 $$^{18}$O的转动发射谱线。通过确定这些分子的丰度，我们的目标是使用衍生的同位素---因此氧同位素----比率，把限制的质量的M型AGB星的样本，还没有被归类为OH/IR星。方法：我们使用详细的辐射传输分析的基础上，加速λ迭代方法来模拟环星分子线发射H$_2$$^{17}$O和H$_2$$^{18}$O的IK Tau，R Dor，W Hya，和R Cas。用于约束我们的模型的发射线来自Herschel/HIFI和Herschel/PACS观测，并且都是光学厚的，这意味着全辐射传输分析是估计分子丰度比的唯一可行方法。结果：我们发现我们的样品中的$^{17}$O/$^{18}$O比值通常较低，范围为0.15至0.69。这与相对较低的初始质量，在范围$sim1.0$至1.5 M$_odot$为每个源，基于恒星演化模型。我们还发现邻-帕拉比接近3，这是预计从温暖的形成预测。结论：在这个样本中发现的$^{17}$O/$^{18}$O比值处于恒星演化模型预测的范围的低端，表明所选择的样本具有相对较低的初始质量。
AIM: In this study we examine rotational emission lines of two isotopologues of water: H$_2$$^{17}$O and H$_2$$^{18}$O. By determining the abundances of these molecules, we aim to use the derived isotopologue --- and hence oxygen isotope --- ratios to put constraints on the masses of a sample of M-type AGB stars that have not been classified as OH/IR stars. METHODS: We use detailed radiative transfer analysis based on the accelerated lambda iteration method to model the circumstellar molecular line emission of H$_2$$^{17}$O and H$_2$$^{18}$O for IK Tau, R Dor, W Hya, and R Cas. The emission lines used to constrain our models come from Herschel/HIFI and Herschel/PACS observations and are all optically thick, meaning that full radiative transfer analysis is the only viable method of estimating molecular abundance ratios. RESULTS: We find generally low values of the $^{17}$O/$^{18}$O ratio for our sample, ranging from 0.15 to 0.69. This correlates with relatively low initial masses, in the range $sim1.0$ to 1.5 M$_odot$ for each source, based on stellar evolutionary models. We also find ortho-to-para ratios close to 3, which are expected from warm formation predictions. CONCLUSIONS: The $^{17}$O/$^{18}$O ratios found for this sample are at the lower end of the range predicted by stellar evolutionary models, indicating that the sample chosen had relatively low initial masses.