Chromospheric activity and photospheric variation of $\alpha$ Ori during the great dimming event in 2020

TL;DR

This study investigates the chromospheric activity and photospheric variations of alpha Ori during the 2020 great dimming event, revealing photospheric causes and long-term activity patterns through spectroscopic and photometric analysis.

Contribution

It provides the first detailed analysis linking photospheric changes to the great dimming, highlighting the role of cool plumes and long-term activity cycles in alpha Ori.

Findings

Photospheric temperature dropped by about 80 K during the dimming.

The Ca II H&K lines' normalized excess flux did not change significantly during the event.

Similar activity patterns occurred during a 1984-1985 dimming event.

Abstract

The so-called great dimming event of alpha Ori in late 2019 and early 2020 sparked our interest in the behaviour of chromospheric activity during this period. To study the timeline of chromospheric activity, we derive a S_MWO time series of TIGRE and Mount Wilson values, and we compare this long time series with photometric data from the AAVSO database. In addition, we determine the absolute and normalised excess flux of the Ca II H&K lines. To do so, we estimate the changing effective temperature from TIGRE spectra and find a clear drop of about 80 K between November 2019 and February 2020, which coincides with the minimum of visual brightness. During the same period, the S-index increased significantly, yet this is a mere contrast effect, because the normalised excess flux of the Ca II H&K lines did not change significantly. However, the latter dropped immediately after this episode. Comparing the combined S_MWO values and visual magnitude time series, we find a similar increase in the S-index during another noticeable decrease in the visual magnitude of alpha Ori, which took place in 1984 and 1985. To also probe the dynamics of the upper photosphere, we analysed the lines in 6251-6263 A and found core distance varies which shows a relation with the great dimming event. This type of variation could be caused by rising and sinking cool plumes as a temporary spill-over of convection on alpha Ori. Based on our study, we conclude that the cause for the great dimming is located in the photosphere. Furthermore, the long-term spectroscopic and photometric time series suggests that this great dimming does not appear to be a unique phenomenon, but rather that such dimmings do occur more frequently, which motivates further monitoring of alpha Ori with facilities such as TIGRE.