An Observational Hertzsprung–Russell Diagram and Fundamental Parameters of Galactic BA Supergiants

Galactic B- and A-type supergiants serve as critical observational anchors for testing massive-star evolution models and calibrating extragalactic distance indicators. Building upon a uniform Teff scale established in our previous work, we construct a self-consistent observational Hertzsprung–Russell diagram (HRD) for a homogeneous sample of 84 Galactic BA supergiants to evaluate their fundamental parameters, evolutionary masses, ages, and evolutionary status. Absolute bolometric luminosities are derived by combining Gaia DR3 astrometry with 3D spatial dust reddening maps and theoretical bolometric corrections. Comparison with Geneva evolutionary tracks for solar metallicity (Z=0.014), incorporating stellar wind mass loss and rotational mixing, demonstrates that the vast majority of the program stars represent massive post-main-sequence objects with initial masses of approximately 7–25M⊙ and ages of ∼8–57 Myr. Our homogeneous analysis also reveals a distinct subgroup of eight low-luminosity objects (log(L/L⊙)<3.5), including stars whose cataloged luminosity classes are inconsistent with supergiant status as well as objects retaining supergiant spectral classifications despite their unexpectedly low derived luminosities. These findings highlight the importance of combining uniform spectroscopic calibrations with Gaia astrometry to refine Galactic supergiant censuses and improve the empirical basis of the flux-weighted gravity–luminosity relation.