长周期变星II中的水汽脉泽。半正则变元R Crt和RT Vir
Water vapour masers in long-period variable stars II. The semi-regular variables R Crt and RT Vir
摘要：
在“Medicina/Effelsberg H2O脉泽监测计划”中，我们已经观察到R Crt和RT Vir的脉泽发射超过二十年。为了了解星周包层中脉泽斑的分布和寿命，我们收集了同一时期的干涉测量数据。我们确认了几个月到1.5年的时间尺度上个别脉泽特征的短时变化。此外，在这两颗恒星上都可以看到独立于个体特征的一般亮度水平的十年变化。这是由于在选定的速度范围内，亮度变化彼此独立地发生，并且与光学光变曲线无关。个别特征的速度的预期漂移通常被混合所掩盖。然而，在RT Vir中，我们发现了一个例外情况，即在7.5年内具有恒定速度（<0.06 km/s/yr）的特征。我们将长期的亮度变化归因于恒星风中密度高于平均值的区域的存在，这些区域中有几个云在短时间尺度上发射微波激射辐射。这些区域通常需要20年才能穿过H2O脉泽壳层，在那里存在激发H2O脉泽的合适条件。恒定速度（11 km/s）的特征很可能来自一个单独的脉泽云，它穿过RT Vir的大约一半的H2O脉泽壳而没有改变速度。由此我们推断，它的路径位于外部的H2O脉泽壳，RT Vir的恒星风显然已经达到其终端流出速度。RT Vir中最高的H2O脉泽流出速度接近OH和CO观测给出的终端流出速度，这一结论得到了证实。这在其他半规则变星中通常是观察不到的。
英文摘要：
Within the 'Medicina/Effelsberg H2O maser monitoring program' we have observed the maser emission of R Crt and RT Vir for more than two decades. To get insight in the distribution and longevity of maser spots in the circumstellar envelopes, we have collected interferometric data, taken in the same period, from the literature. We confirm short-time variations of individual maser features on timescales of months to up to 1.5 years. Also decade-long variations of the general brightness level independent from individual features were seen in both stars. These are due to brightness variations occurring independently from each other in selected velocity ranges, and are independent of the optical lightcurves. Expected drifts in velocity of individual features are usually masked by blending. However, in RT Vir we found an exceptional case of a feature with a constant velocity over 7.5 years (<0.06 km/s/yr). We attribute the long-term brightness variations to the presence of regions with higher-than-average density in the stellar wind, which host several clouds which emit maser radiation on the short time scales. These regions typically need ~20 years to cross the H2O maser shell, where the right conditions to excite H2O masers are present. The constant velocity feature (11 km/s) is likely to come from a single maser cloud, which moved through about half of RT Vir's H2O maser shell without changing velocity. From this we infer that its path was located in the outer part of the H2O maser shell, where RT Vir's stellar wind apparently has already reached its terminal outflow velocity. This conclusion is corroborated by the observation that the highest H2O maser outflow velocity in RT Vir approaches the terminal outflow velocity as given by OH and CO observations. This is generally not observed in other semi-regular variable stars.
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