长周期变星中的水汽脉泽。I. RX Boo和SV Peg
Water vapour masers in long-period variable stars. I. RX Boo and SV Peg
摘要：
我们希望了解晚型恒星拱壳层中H2O脉泽强变率的原因。我们用单碟望远镜监测了22 GHz水蒸气脉泽谱线中的RX Boo和SV Peg。RX Boo的监测期为20年（1987 - 2007），SV Peg的监测期为12年（1990 - 1995，2000 - 2007）。此外，还获得了RX Boo在几年内使用超大型阵列的地图。我们发现RX Boo星周壳层中的大部分辐射位于一个不完整的环中，其内径为91质量（15 AU）。在NW-SE方向上存在速度梯度。脉泽区可以模拟为一个厚度为22 AU的壳层，其中只有部分填充。天然气穿越时间为16.5年。环形结构和速度梯度保持稳定至少11年，而脉泽线轮廓变化强烈。这表明，空间的不对称性不是偶然的，因此，无论是质量损失过程或微波激射器的激发条件在RX博不是球对称的。脉泽光谱特征的强烈变化主要是由于脉泽发射斑点的非相干强度波动，其寿命为1年。我们没有发现任何恒星的光学和脉泽变化之间的相关性。SV Peg脉泽的可变性与RX Boo脉泽的可变性没有实质性的不同。光谱特征较少，变化范围较窄。微波激射器仅在1990 - 1992年和2006/2007年活跃于>10-Jy水平。在其他时候，脉泽要么不存在（<1 Jy），要么几乎检测不到。
英文摘要：
We wish to understand the reasons for the strong variability of H2O masers in circumstellar shells of late-type stars. We monitored RX Boo and SV Peg in the 22-GHz maser line of water vapour with single-dish telescopes. The monitoring period covered two decades for RX Boo (1987 -- 2007) and 12 years for SV Peg (1990 -- 1995, 2000 -- 2007). In addition, maps were obtained of RX Boo with the Very Large Array over several years. We find that most of the emission in the circumstellar shell of RX Boo is located in an incomplete ring with an inner radius of 91 mas (15 AU). A velocity gradient is found in a NW--SE direction. The maser region can be modelled as a shell with a thickness of 22 AU, which is only partially filled. The gas crossing time is 16.5 years. The ring-like structure and the velocity gradient remained stable for at least 11 years, while the maser line profiles varied strongly. This suggests that the spatial asymmetry is not accidental, so that either the mass loss process or the maser excitation conditions in RX Boo are not spherically symmetric. The strong variability of the maser spectral features is mainly due to incoherent intensity fluctuations of maser emission spots, which have lifetimes of the order of 1 year. We found no correlation between the optical and the maser variability in either star. The variability properties of the SV Peg masers do not differ substantially from those of RX Boo. There were fewer spectral features present, and the range of variations was narrower. The maser was active on the >10-Jy level only 1990 -- 1992 and 2006/2007. At other times the maser was either absent (<1 Jy) or barely detectable.
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