The Metallicity and Distance of Leo A from Blue Supergiants

Abstract

We have obtained high-quality spectra of blue supergiant candidates in the dwarf irregular galaxy Leo A with the Low Resolution Imaging Spectrometer at the Keck I telescope. From the quantitative analysis of seven B8–A0 stars, we derive a mean metallicity [Z] = −1.35 ± 0.08, in excellent agreement with the gas-phase chemical abundance. From the stellar parameters and the flux-weighted gravity–luminosity relation (FGLR), we derive a spectroscopic distance modulus m − M = 24.77 ± 0.11 mag, significantly larger (∼0.4 mag) than the value indicated by RR Lyrae and other stellar indicators. We explain the bulk of this discrepancy with blue loop stellar evolution at very low metallicity and show that the combination of metallicity effects and blue loop evolution amounts, in the case of Leo A, to an ∼0.35 mag offset of the FGLR to fainter bolometric luminosities. We identify one outlier of low bolometric magnitude as a post-AGB star. Its metallicity is consistent with that of the young population, confirming the slow chemical enrichment of Leo A.