Variability of Blue Supergiants in the LMC with TESS

Abstract

The blue supergiant (BSG) problem, namely, the overabundance of BSGs inconsistent with classical stellar evolution theory, remains an open question in stellar astrophysics. Several theoretical explanations have been proposed, which may be tested by their predictions for the characteristic time variability. In this work, we analyze the light curves of a sample of 20 BSGs obtained from the Transiting Exoplanet Survey Satellite (TESS) mission. We report a characteristic signal in the low-frequency (f ≲ 2 day−1) range for all our targets. The amplitude spectrum has a peak frequency of ∼0.2 day−1, and we are able to fit it by a modified Lorentzian profile. The signal itself shows strong stochasticity across different TESS sectors, suggesting its driving mechanism happens on short (≲months) timescales. Our signals resemble those obtained for a limited sample of hotter OB stars and yellow supergiants, suggesting their possible common origins. We discuss three possible physical explanations: stellar winds launched by rotation, convection motions that reach the stellar surface, and waves from the deep stellar interior. The peak frequency of the signal favors processes related to the convective zone caused by the iron opacity peak, and the shape of the spectra might be explained by the propagation of high-order, damped gravity waves excited from that zone. We discuss the uncertainties and limitations of all these scenarios.