The metallicity and distance of Leo A from blue supergiants

TL;DR

This study uses spectra of blue supergiants in Leo A to determine its metallicity and distance, revealing a lower metallicity consistent with gas-phase measurements and a larger distance than previous stellar indicators, influenced by stellar evolution effects.

Contribution

It provides the first spectroscopic metallicity measurement of Leo A's blue supergiants and investigates the impact of stellar evolution on distance estimates using the FGLR method.

Findings

Metallicity [Z] = -1.35 ± 0.08 consistent with gas-phase abundance.

Spectroscopic distance modulus m-M = 24.77 ± 0.11 mag, larger than previous estimates.

Blue loop stellar evolution at low metallicity causes a ~0.35 mag offset in FGLR.

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-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 a ~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.