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arXiv 1409.0396astro-ph.SR
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Herschel看到的W Hya的风。II。W Hya的分子被膜

The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya

T. Khouri, A. de Koter, L. Decin, L. B. F. M. Waters, M. Maercker, R. Lombaert, J. Alcolea, J. A. D. L. Blommaert, V. Bujarrabal, M. A. T. Groenewegen, K. Justt… 展开作者

T. Khouri, A. de Koter, L. Decin, L. B. F. M. Waters, M. Maercker, R. Lombaert, J. Alcolea, J. A. D. L. Blommaert, V. Bujarrabal, M. A. T. Groenewegen, K. Justtanont, F. Kerschbaum, M. Matsuura, K. M. Menten, H. Olofsson, P. Planesas, P. Royer, M. R. Schmidt, R. Szczerba, D. Teyssier, J. Yates

摘要:

渐近巨星分支(AGB)上的中低质量恒星的演化主要受这些恒星在恒星风中失去质量的速率控制。了解AGB星星风的驱动机制和强度以及核合成产物在其表面富集的过程对于约束AGB演化和预测星系的化学演化至关重要。在以前的论文中,我们已经限制了结构的流出包络的W Hya使用光谱线的$^{12}$ CO分子。在这里,我们扩大了这项研究的建模广泛的一组H $_{2}$ O和$^{28}$ SiO线观察到的三台仪器上赫歇尔使用国家的最先进的分子激发和辐射传输代码。氧同位素比值和$^{28}$ SiO丰度分布可以分别与初始恒星质量和恒星风底部尘埃形成的关键方面联系起来。H $_{2}$ O和$^{28}$ SiO的模拟证实了由$^{12}$ CO线导出的W Hya包络模型的性质。我们发现一个H $_2$ O ortho-to-para比为2.5\,$^{+2.5}_{-1.0}$,符合预期的AGB风。O $^{16}$ /O $^{17}$比表明W Hya的初始质量约为1.5 M $_\odot$。虽然HIFI观测到的邻位和对位H $_{2}$ O线似乎是物理性质略有不同的示踪气体,但$0.7\pm0.1$ km·s $^{-1}$的湍流速度与两种自旋异构体和$^{28}$ SiO的HIFI线吻合得很好。邻位和对位H $_2^{16}$ O和$^{28}$ SiO相对于H $_{2}$的丰度分别为$(6^{+3}_{-2}) \times 10^{-4}$、$(3^{+2}_{-1}) \times 10^{-4}$和$(3.3\pm 0.8)\times 10^{-5}$。假设太阳的硅碳比,$^{28}$ SiO线发射模型与大约三分之一的硅原子被锁定在尘埃颗粒中是一致的。

英文摘要:

The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the $^{12}$CO molecule. Here we broaden this study by modelling an extensive set of H$_{2}$O and $^{28}$SiO lines observed by the three instruments on board Herschel using a state-of-the-art molecular excitation and radiative transfer code. The oxygen isotopic ratios and the $^{28}$SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. The modelling of H$_{2}$O and $^{28}$SiO confirms the properties of the envelope model of W Hya derived from $^{12}$CO lines. We find an H$_2$O ortho-to-para ratio of 2.5\,$^{+2.5}_{-1.0}$, consistent with what is expected for an AGB wind. The O$^{16}$/O$^{17}$ ratio indicates that W Hya has an initial mass of about 1.5 M$_\odot$. Although the ortho- and para-H$_{2}$O lines observed by HIFI appear to trace gas of slightly different physical properties, a turbulence velocity of $0.7\pm0.1$ km s$^{-1}$ fits the HIFI lines of both spin isomers and those of $^{28}$SiO well. The ortho- and para-H$_2^{16}$O and $^{28}$SiO abundances relative to H$_{2}$ are $(6^{+3}_{-2}) \times 10^{-4}$, $(3^{+2}_{-1}) \times 10^{-4}$, and $(3.3\pm 0.8)\times 10^{-5}$, respectively. Assuming a solar silicon-to-carbon ratio, the $^{28}$SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.

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