Variability of Galactic blue supergiants observed with TESS

Abstract

Blue supergiants (BSGs) mediate between the main sequence and the late stages of massive stars, which makes them valuable for assessing the physics that drives the stars across the diverse evolutionary channels. By exploring correlations between the parameters of BSGs and their variability properties, we aim to improve the constraints on the models of the evolved star structure and on the physics of the post-main-sequence evolution. We conducted a variability study of 41 BSGs with known spectroscopic parameters in the Galaxy using photometry from the Transiting Exoplanet Survey Satellite. We described the time domain of the stars by means of three statistical measures and extracted frequencies via iterative pre-whitening. Alongside, we investigated the stochastic low-frequency (SLF) variability, which manifests itself in all amplitude spectra. We report a positive correlation between the amplitude of variability and the stellar luminosity. For log(L/L${}_{\odot }\lesssim 5$), stars display frequencies that match the rotational one, suggesting that variability is driven by surface spots and/or features embedded in the wind. For log(L/L${}_{\odot }\gtrsim 5$), variables of the $\alpha$ Cyg class manifest themselves with their diverse and/or time-variant properties. Moreover, we report a positive correlation between the SLF variability amplitude and $T_{\mathrm{eff}}$, indicating an influential role that the stellar age plays on the emergence of the background signal beyond the main sequence. Positive, though weak, correlation is observed between the intrinsic brightness and the SLF variability amplitude, similar to the findings in the Large Magellanic Cloud, which suggests an in common excitation mechanism that depends only mildly on metallicity. Exceptionally, the $\alpha$ Cyg variables display a suppressed SLF variability that prompts to the interior changes that the evolving stars undergo.

26 pages (incl. appendices), 10 figures in the main text. The appendices include 6 figures, 4 tables. Accepted for publication in A&A

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