2025ApJ...985..150M

As JWST uncovers increasingly strong evidence that metal-poor, massive stars in early galaxies dominated reionization, observational constraints on the properties of such stars are more relevant than ever before. However, spectra of individual O- and B-type stars are rare at the relevant metallicities (≲0.2Z_⊙), leaving models of stellar evolution and ionizing flux poorly constrained by data in this regime. We present new medium-resolution (R ∼ 4000) optical spectra of 17 OB stars in the local low-metallicity (0.12 Z_⊙) dwarf galaxy NGC 3109. We assign spectral types to the stars and present new criteria for selecting O stars using optical and near-UV photometry from Hubble Space Telescope imaging. We fit the spectra and photometry with grids of stellar atmosphere models to measure stellar temperatures, surface gravities, luminosities, radii, and masses. We find evidence of strong mass loss via radiation-driven stellar winds in two O stars, one of which is the hottest, youngest, and most massive star confirmed in the host galaxy to date. Though its spectrum does not meet conventional Wolf-Rayet spectral classification criteria, this metal-poor O5 If star produces strong He IIλ4686 emission, and its evolutionary status is ambiguous. This work nearly doubles the number of OB stars with measured parameters in NGC 3109, including 12 stars with no previously reported parameters, five with no published spectroscopy, and four binary candidates. This large sample of OB stellar parameters provides a new observational testbed to constrain the stellar astrophysics that drove cosmic reionization and influenced the evolution of the earliest galaxies.