长周期变星中的水蒸气脉泽 III. 米拉变星 U Her 和 RR Aql
Water vapour masers in long-period variable stars III. Mira variables U Her and RR Aql
AI总结：
本文通过约二十年22 GHz监测及VLA成图，研究米拉变星 U Her 与 RR Aql 的水蒸气脉泽，揭示其壳层结构、云寿命及多时间尺度亮度变化机制。
摘要：
在“Medicina/Effelsberg H2O 脉泽监测计划”框架内，我们于1990年至2011年间在22 GHz频率上对 U Her 和 RR Aql 进行了约二十年的观测；其中 RR Aql 在1997年至2000年间存在观测间断。此外，我们还在1990年至1992年期间利用甚大阵（Very Large Array）获得了 U Her 的图谱。我们发现，U Her 中最强的发射位于一个边界为11–25 AU 的壳层内。气体穿越时间为8.5年。基于脉泽发射未表现出可探测的视向速度漂移，我们推断单个脉泽云的寿命短于4年。该壳层并未被均匀填充，而且其结构得以维持的时间尺度远长于单个脉泽云的寿命。两颗恒星都在多个时间尺度上表现出亮度变化。主要变化是周期性的，跟随恒星的光学变化，但存在2–3个月的滞后。除此之外，还叠加有持续数月、发生在特定位置的增强或减弱激发的不规则涨落，以及十年或更长时间尺度上的长期系统性变化。脉泽发射的性质由恒星风穿过水脉泽壳层时的性质所决定。恒星风中影响激发条件和主要束射方向的非均匀性，很可能导致了在长于恒星脉动周期的时间尺度上观测到的变化。我们提出，壳层中存在长寿命区域，这些区域在恒星风穿越壳层的时间尺度上，或在（亚）恒星伴星的轨道周期时间尺度上，维持有利的激发条件。
英文摘要：
Within the 'Medicina/Effelsberg H2O maser monitoring program' we observed U Her and RR Aql at 22-GHz for about two decades between 1990 and 2011, with a gap between 1997 and 2000 in the case of RR Aql. In addition, maps were obtained in the period 1990-1992 of U Her with the Very Large Array. We find that the strongest emission in U Her is located in a shell with boundaries 11-25 AU. The gas crossing time is 8.5 years. We derive lifetimes for individual maser clouds of less than 4 years, based on the absence of detectable line-of-sight velocity drifts of the maser emission. The shell is not evenly filled, and its structure is maintained on timescales much longer than those of individual maser clouds. Both stars show brightness variability on several timescales. The prevalent variation is periodic, following the optical variability of the stars with a lag of 2-3 months. Superposed are irregular fluctuations, of a few months' duration, of increased or decreased excitation at particular locations, and long-term systematic variations on timescales of a decade or more. The properties of the maser emission are governed by those of the stellar wind while traversing the water maser shell. Inhomogeneities in the wind affecting the excitation conditions and prevalent beaming directions likely cause the variations seen on timescales longer than the stellar pulsation period. We propose the existence of long-living regions in the shells, which maintain favourable excitation conditions on timescales of the wind crossing times through the shells or orbital periods of (sub-)stellar companions.
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