2019-2020年参宿四的差分散斑偏振：上升与下降不同
Differential Speckle Polarimetry of Betelgeuse in 2019-2020: the rise is different from the fall
摘要：
最近发表的对参宿四的情景光谱（Levesque and Massey，2020）和高角分辨率（Montarges et al，2020）观测表明，2019-2020年的深度极小值是由恒星大气中尘埃丰度增加引起的。对这些事件的详细监测可能有助于构建演化恒星质量损失的一致物理模型。对于这样的观测来说，采用分辨不均匀恒星大气的方法是至关重要的。我们介绍了在SAI MSU高加索山天文台的2.5米望远镜上对参宿四的差分散斑偏振观测，覆盖了2019-2020年的最小值。这些观测是在465、550、625和880 nm波长下进行的。在所有日期和所有波长上都探测到角大小为$\approx0.1^{\prime\prime}$的星周反射星云。在观测期间，星云的形态发生了显著的变化。直到2020年2月，包层的净偏振亮度保持不变，而恒星$V$带通量减少了2.5倍。从2020年2月中旬开始，包层的偏振通量上升了2.1倍，与此同时，恒星恢复到了2019年10月的前极小状态。根据这些数据和我们在2020年4月6日获得的低分辨率光谱，我们确认了一个结论，即最小值是由位于视线上的尘埃云形成引起的。当使用关于其热辐射的数据时，将有可能对这种云进行定量描述。
英文摘要：
Recently published episodic spectral (Levesque and Massey, 2020) and high angular resolution (Montarges et al, 2020) observations of Betelgeuse suggest that the deep minimum of 2019-2020 was caused by an enhanced dust abundance in the stellar atmosphere. Detailed monitoring of such events may prove useful for constructing consistent physical models of mass loss by evolved stars. For such observations it is fundamentally important to employ methods resolving an inhomogeneous stellar atmosphere. We present the differential speckle polarimetric observations of Betelgeuse at 2.5-m telescope of Caucasian Mountain Observatory of SAI MSU covering the period of 2019-2020 minimum. The observations were secured on 17 dates at wavelengths 465, 550, 625 and 880 nm. The circumstellar reflection nebula with the angular size of $\approx0.1^{\prime\prime}$ was detected for all the dates and at all wavelengths. The morphology of the nebula changed significantly over the observational period. Net polarized brightness of the envelope remained constant until February 2020, while the stellar $V$ band flux decreased 2.5 times. Starting from mid-February 2020, polarized flux of the envelope rose 2.1 times, at the same time the star returned to the pre-minimum state of October 2019. Basing on these data and our low resolution spectrum obtained on 2020-04-06 we confirm a conclusion that the minimum is caused by the formation of a dust cloud located on the line of sight. A quantitative characterisation of this cloud will be possible when the data on its thermal radiation are employed.
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