论文标题
长周期变化星中的水蒸气masers II。半规则变量R CRT和RT VIR
Water vapour masers in long-period variable stars II. The semi-regular variables R Crt and RT Vir
论文作者
论文摘要
在“ Medicina/Effelsberg H2O MASER监测计划”中，我们已经观察到R CRT和RT VIR的Maser发射已有二十多年了。为了洞悉Maser斑点在情节信封中的分布和寿命，我们从同一时期收集了来自文献的干涉数据。 我们确认在最多1.5岁的时间尺度上，单个MASER特征的短期变化。在两颗恒星中都可以看到一般亮度水平的十年变化与单个特征无关。这些是由于亮度变化在选定的速度范围内彼此独立发生，并且独立于光学曲面。通常通过混合掩盖了单个特征速度的预期漂移。但是，在RT VIR中，我们发现了一个特征的特殊情况，其恒定速度超过7。5年（<0.06 km/s/yr）。 我们将长期的亮度变化归因于恒星风中高度高于平均水平的区域的存在，恒星风中有几个云，这些云在短时间尺度上发出了maser辐射。这些地区通常需要约20年才能越过H2O Maser Shell，在那里有适当的条件引起了H2O Masers。恒定速度特征（11 km/s）可能来自单个maser云，该云通过RT Vir的H2O Maser壳的大约一半移动而不会改变速度。由此我们推断出它的路径位于H2O Maser Shell的外部，RT Vir的恒星风显然已经达到了其末端流出速度。通过观察到，RT ViR中最高的H2O MASER流出速度接近OH和CO观测值的末端流出速度，这一结论得到了证实。通常在其他半定期变量恒星中观察到这一点。
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.
