LMC Blue Supergiant Stars and the Calibration of the Flux-weighted Gravity–Luminosity Relationship

Abstract

High-quality spectra of 90 blue supergiant stars in the Large Magellanic Cloud are analyzed with respect to effective temperature, gravity, metallicity, reddening, extinction, and extinction law. An average metallicity, based on Fe and Mg abundances, relative to the Sun of [ Z ] = −0.35 ± 0.09 dex is obtained. The reddening distribution peaks at = 0.08 mag, but significantly larger values are also encountered. A wide distribution of the ratio of extinction to reddening is found ranging from = 2 to 6. The results are used to investigate the blue supergiant relationship between flux-weighted gravity, g f ≡ g / , and absolute bolometric magnitude M bol . The existence of a tight relationship, the Flux-weighted Gravity–Luminosity Relationship (FGLR), is confirmed. However, in contrast to previous work, the observations reveal that the FGLR is divided into two parts with a different slope. For flux-weighted gravities larger than 1.30 dex, the slope is similar to that found in previous work, but the relationship becomes significantly steeper for smaller values of the flux-weighted gravity. A new calibration of the FGLR for extragalactic distance determinations is provided.