Analysis of photometric and spectroscopic variability of red supergiant Betelgeuse

Abstract

Betelgeuse is a pulsating red supergiant whose brightness is semi periodically variable and in February 2020 reached a historical minimum, the Great Dimming. The aims of this study are to characterize Betelgeuse's variability based on available archival data and to study possible causes of light variability. Many spectra, from ultraviolet and optical regions, were evaluated for spectral analysis. The spectra were used primarily to determine radial velocities from different layers of atmosphere and their long{-}term evolution. Additionally, photometric data were analyzed in different filters as well, to construct light curves and to determine periods of the variability. Spectroscopic and photometric variability are compared to each other and given into a context with the Great Dimming. The two most dominant photometric periods are $P_1=2190\pm 270\mathrm{d}$ and $P_2=417\pm 17\mathrm{d}$, while the dominant optical radial velocity periods are $P_{1,v_{\mathrm{r}}}=2510\pm 440\mathrm{d}$ and $P_{2,v_{\mathrm{r}}}=415\pm 11\mathrm{d}$. In the same time, the radial velocity determined from ultraviolet spectra also shows variability and is distinctively different from the variability of photospheric velocity, undergoing longer periods of variability. We attribute these velocities to the velocities at the base of outflowing wind. We also report a maximum of stellar wind velocity during the Great Dimming, accompanied by the previously reported minimum of brightness and the maximum of photospheric radial velocity. After the Dimming, Betelgeuse mode of variability has fundamentally changed and is now instead following a shorter period of $\sim 200\mathrm{d}$.

12 pages, 4 figures, 5 tables

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