On the evolutionary nature of massive B-type supergiants: a modern empirical reappraisal using data from IACOB, Gaia and TESS

Abstract: Massive stars are key contributors to the chemodynamical evolution of galaxies and the Universe. Despite their significance, discrepancies between observational data and theoretical models of massive stars challenge our understanding of these objects. A major uncertainty is the overdensity of B-type supergiants (BSGs) in the Hertzsprung-Russell diagram, where models predict the end of the main sequence phase (or TAMS). Is uncertain whether the TAMS needs to be redefined or if the overdensity results from overlapping populations following different evolutionary paths. Conceived as direct descendants of O-type stars, BSGs may include stars not only evolving in the main sequence but also returning from a post-red supergiant phase. A representative fraction of massive stars are predicted to be products of binary interaction, creating additional evolutionary channels. In addition, some fundamental properties of BSGs such as the spin- and mass-loss rates are not as well constrained as in O-type stars, having a significant impact on massive star evolution. To overcome this situation, statistically significant spectroscopic samples offer a unique opportunity to study the physical and chemical properties of BSGs. Moreover, the advent of space astrometry and photometry missions such as Gaia and TESS has brought a new era for studying additional properties in detail. This thesis comprises the study of 1000 Galactic blue supergiants (O- and B-type) combining multi-epoch high-resolution spectroscopic data from the IACOB project and the ESO archive with Gaia distances and TESS photometry, becoming the largest holistic empirical study of the physical, chemical, and pulsational properties of these objects performed to date. All these properties gathered into a unique volume-limited sample allowed to provide an empirical reassessment of the main properties of BSGs and investigate their intricate nature.