Characterisation of the hyperactive star HD 251108: Spectroscopic monitoring and discovery of optical superflares

Giant stars can be magnetically active. A prominent example is HD 251108, which exhibited a giant flare recorded on 7 November 2022 at X-ray and optical wavelengths. We report on a spectroscopic campaign monitoring this star from November 2022 to March 2024, starting with the late phases of the giant flare and accumulating 149 medium-resolution spectra with the TIGRE telescope. HD 251108 shows periodic variations in radial velocity (RV), which we interpret as being caused by the star being part of a binary system, yet the photospheric lines do not show any obvious signs of a second component. Similar periodic changes are found in the effective temperature and in the equivalent widths (EWs) of various chromospheric emission lines with the same period as observed in RV, but with a phase shift. We find these effects could be caused by a plage rotating with the star if the star is rotating synchronously. Since the star shows strong Hα and even He I infrared triplet emission outside the flare, we find it to be hyperactive, even though the star only has moderate lithium abundance of A(Li) = 1.28 dex. We further investigated archival TESS observations of HD 251108 and found that flares with energy releases in excess of 1038 erg at optical wavelengths are not unusual. Finally, considering the November 2022 event in a broader context, we argue that this event likely released more than 1040 erg at optical wavelengths.