Variability of Galactic blue supergiants observed with TESS

Abstract

Context. Blue supergiants (BSGs) span phases between the main sequence and the late stages of massive stars, which makes them valuable for assessing the physics that drives stars across diverse evolutionary channels. Aims. By exploring correlations between the parameters of BSGs and their variability properties, we aim to improve the constraints on models of the evolved star structure and on the physics of post-main-sequence evolution. Methods. We conducted a variability study of 41 BSGs with known spectroscopic parameters in the Galaxy using high-precision photometry from the Transiting Exoplanet Survey Satellite. Stellar luminosities were calculated from the fit of multiband photometry and using the latest distance estimates from Gaia. We described the time domain of the stars by means of three statistical measures and extracted prominent frequencies via an iterative pre-whitening process. We also investigated the debated stochastic low-frequency (SLF) variability, which manifests itself in all amplitude spectra. Results. We report a positive correlation between the amplitude of photometric variability and the stellar luminosity. For log (L/L ) . 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 ) & 5, variables of the α Cygni class manifest themselves via their diverse and/or time-variant photometric properties and their systematically lower frequencies. Moreover, we report a positive correlation between the SLF variability amplitude and the effective temperature, which indicates that the stellar age plays an influential role in the emergence of the background signal beyond the main sequence. A positive, though weak, correlation is also observed between the intrinsic brightness and the SLF variability amplitude, similar to the findings in the Large Magellanic Cloud, which suggests an excitation mechanism that depends only mildly on metallicity. Exceptionally, the α Cygni variables display a suppressed SLF variability that points to the interior changes that the evolving stars undergo.