[论文解读] Water vapour masers in long-period variable stars II. The semi-regular variables R Crt and RT Vir

J. Brand, D. Engels+1 more|arXiv (Cornell University)|Oct 31, 2020

Stellar, planetary, and galactic studies参考文献 45被引用 6

一句话总结

本研究利用超过二十年的单天线监测数据及档案干涉测量数据，调查了半规则变星 R Coronae Australis（R Crt）和 R Sculptoris（RT Vir）中22 GHz水激微波脉泽的长期与短期变异性。研究发现，长期亮度变化（十年量级）源于包围恒星的包层中密集且不均匀的区域，这些区域包含多个寿命短暂的脉泽云团；而RT Vir中一个稳定的11 km s−1特征表明其在外层壳层中以终端风速运动，暗示存在寿命达数十年的局域高密度结构。

ABSTRACT

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.

研究动机与目标

• 理解演化恒星包层中H2O脉泽发射的起源与演化。
• 研究半规则变星（SRb型）脉泽发射亮度变化的时间尺度与成因。
• 通过干涉测量数据将单天线谱变异性与脉泽斑点的空间分布联系起来。
• 确定脉泽变异性是否与光学光曲线相关，或由恒星风的内在不均匀性驱动。
• 评估恒星风结构与密度增强在塑造脉泽发射模式中的作用。

提出的方法

• 利用Medicina和Effelsberg射电望远镜对22 GHz H2O脉泽发射进行长达20余年的长期单天线监测。
• 收集并分析文献中的干涉测量数据，以绘制R Crt和RT Vir中脉泽斑点的空间分布与运动学特征。
• 将单天线光曲线与干涉测量图进行互相关分析，以识别速度偏移与混合效应。
• 使用GILDAS软件进行数据降噪与谱分析，以提取通量密度与速度演化特征。
• 将脉泽速度偏移与基于CO和OH观测推导出的恒星风终端速度进行比较。
• 对不同速度通道的脉泽亮度变化进行统计分析，以识别独立于单个特征的长期趋势。

实验结果

研究问题

• RQ1 在R Crt和RT Vir中，H2O脉泽发射的十年量级亮度变化（独立于单个谱线特征）的成因是什么？
• RQ2 持续数月到1.5年的短期脉泽波动如何与单个脉泽云团的动力学相关？
• RQ3 为何RT Vir中的11 km s−1特征在7.5年内未表现出速度偏移，这对其在包层中位置有何含义？
• RQ4 脉泽亮度变化在多大程度上与中心恒星的光学光曲线相关？
• RQ5 恒星风中的不均匀性如何解释观测到的脉泽变异性以及某些特征的表观长期存在？

主要发现

• R Crt和RT Vir中的长期亮度变化时间尺度为数十年，由不同速度范围内的独立亮度变化驱动，且与光学光曲线无关。
• 持续数月到1.5年的短期脉泽波动归因于单个脉泽云团的瞬时出现与衰减。
• RT Vir中11 km s−1的特征在7.5年内未出现速度偏移（< 0.06 km s−1 yr−1），表明其源自一个稳定存在的脉泽云团，以终端风速在外层壳层中运动。
• 长期稳定特征的存在可由包层中密集且高密度的区域来解释，这些区域容纳了多个寿命短暂的脉泽云团，其典型穿越时间为20年。
• RT Vir中最高的脉泽喷流速度接近终端风速，这与CO和OH观测结果一致，表明脉泽发射产生于风的外层加速区域。
• R Crt和RT Vir的平均脉泽光度比RX Boo和SV Peg高出一个数量级，但这一差异与恒星光度或质量损失率无关，表明其他因素（如局部风的不均匀性）主导了脉泽活动。