关于人马座VX的性质：它是一颗TZO、一颗RSG还是一颗高质量的AGB恒星？
On the nature of VX Sagitarii: Is it a TZO, a RSG or a high-mass AGB star?
摘要：
目标。我们提出了一个光谱分析的基础上，结合AAVSO光变曲线数据的高分辨率观测的非常明亮的红色恒星VX Sgr。鉴于VX Sgr令人困惑的特点，我们探讨了三种情况下，它的性质：一个巨大的红超巨星（RSG）或超巨星（RHG），索恩Zytkow对象（TZO），和极端的渐近巨星分支（AGB）的明星。方法.采样一个以上的整个周期的光度变化，我们得出恒星大气参数使用国家的最先进的PHOENIX大气模型。我们将它们与光学和近红外光谱类型进行比较。我们报告的一些关键功能，由于中性元素原子物种，如Li I，Ca I和Rb I。结果我们提供了新的见解，其亮度，演化阶段以及脉动周期。基于所有的数据，有两个强有力的理由相信VX Sgr是某种极端的AGB星。首先，它在活跃阶段的类似Mira的行为。VX Sgr显示光的变化，其幅度比任何已知的RSG都大得多，并且明显大于所有的RHG。此外，它显示巴耳末线发射，并在这里显示的第一次，线加倍的金属光谱在最大的光，这两个特点典型的Miras。其次，与任何已知的RSG或RHG不同，VX Sgr显示强Rb I线。除了有时可以看到的光球线，它总是显示拱星组件的膨胀速度是兼容的包层中的OH脉泽，证明了一个连续的丰富的外层大气与s-过程元素，一个行为，只能解释为第三疏浚在热脉冲阶段。
英文摘要：
Aims. We present a spectroscopic analysis of the extremely luminous red star VX Sgr based on high-resolution observations combined with AAVSO light curve data. Given the puzzling characteristics of VX Sgr, we explore three scenarios for its nature: a massive red supergiant (RSG) or hypergiant (RHG), a Thorne Zytkow object (TZO), and an extreme Asymptotic Giant Branch (AGB) star. Methods. Sampling more than one whole cycle of photometric variability, we derive stellar atmospheric parameters by using state-of-the-art PHOENIX atmospheric models. We compare them to optical and near infrared spectral types. We report on some key features due to neutral elemental atomic species such as Li I, Ca I, and Rb I. Results. We provide new insights into its luminosity, evolutionary stage as well as its pulsation period. Based on all the data, there are two strong reasons to believe that VX Sgr is some sort of extreme AGB star. Firstly, its Mira-like behaviour during active phases. VX Sgr shows light variations with amplitudes much larger than any known RSG and clearly larger than all RHGs. In addition, it displays Balmer line emission and, as shown here for the first time, line doubling of its metallic spectrum at maximum light, both characteristics typical of Miras. Secondly, unlike any known RSG or RHG, VX Sgr displays strong Rb I lines. In addition to the photospheric lines that are sometimes seen, it always shows circumstellar components whose expansion velocity is compatible with that of the OH masers in the envelope, demonstrating a continuous enrichment of the outer atmosphere with s-process elements, a behaviour that can only be explained by third dredge up during the thermal pulse phase.
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