2023A&A...669A...9M

Aims. The so-called great dimming event of α Ori in late 2019 and early 2020 sparked our interest in the behaviour of chromospheric activity during this period. α Ori was already part of the long-term monitoring program of our TIGRE telescope to study the stellar activity of giant stars, and therefore regular measurements of α Ori have been taken since 2013.
Methods. In the context of this study, we determined the TIGRE S -index values and, using a set of calibration stars, converted these to the Mount Wilson S -index scale, which allows us to combine our TIGRE activity measurements with the S_MWO values taken during the landmark Mount Wilson program some decades earlier and to compare that extended time series with the visual and V magnitude photometric data from the AAVSO database. In addition, we determined the absolute and normalised excess flux of the Ca II H&K lines. To understand the activity in absolute terms, we also assessed the changes in effective temperature using the TiO bands covered by our TIGRE spectra.
Results. We find a clear drop in effective temperature by about 80 K between November 2019 and February 2020, which coincides with the minimum of visual brightness. In addition, the effective (luminous) photospheric area of α Ori also shrank. This might be related to a temporary synchronisation of several large convective cells in cooling and sinking down. 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 α Ori which took place in 1984 and 1985. These two episodes of dimming therefore seem to share a common nature. To probe the dynamics of the upper photosphere, we further analysed the closely neighbouring lines of V I and Fe I at 6251.82 and 6251.56 Å respectively. Remarkably, their core distance varies, and once converted to radial velocity, shows a relation with the great dimming event, as well as with the consecutive, weaker dimming episode in the observing season of 2020 and 2021. This type of variation could be caused by rising and sinking cool plumes as a temporary spill-over of convection on α Ori.
Conclusions. As the effective temperature of α Ori is variable, the S -index, computed relative to a near-ultraviolet (NUV) continuum, is only of restricted use for any monitoring study of the chromospheric activity of α Ori. It is therefore important to consider the effective temperature variability and derive the normalised Ca II H&K flux to study the chromospheric long-term changes in absolute terms. In fact, the Ca II H&K normalised excess flux time series shows that the chromospheric emission of α Ori did not change significantly between November 2019 and February 2020, but then beyond the great dimming minimum it does vary. Hence, this delay of the chromospheric reaction suggests that the cause for the great dimming is located in the photosphere. An investigation of the long-term spectroscopic and photometric time series of α Ori suggests that the great dimming in 2019 and 2020 does not appear to be a unique phenomenon, but rather that such dimmings do occur more frequently, which motivates further monitoring of α Ori with facilities such as TIGRE.