Extremely iron-poor O-type stars in the Magellanic Bridge

Context. To study stars analogous to those in the early Universe with redshift z > 3, we need to probe environments with low metallicities. Until recently, massive O-type stars with a metallicity lower than that of the Small Magellanic Cloud (SMC; Z < 20% Z⊙) were only known in compact dwarf galaxies. The signal-to-noise ratio and spatial resolution of observations of stars in such distant galaxies (> 1 Mpc) are limited. Recently, a few O-type stars were identified in the nearby Magellanic Bridge. This is a unique laboratory with a low gas density and low metal content. Aims. We acquired high-resolution HST-COS far-UV spectra of two O-type stars in the Magellanic Bridge. Using the UV forest of iron lines from these observations, we measured the inherent iron abundances precisely and determined the metallicity of the stars. Methods. Using detailed expanding nonlocal thermal equilibrium atmosphere models, we generated synthetic spectra for different iron abundances and for a range of microturbulent velocities. We used Bayesian posterior sampling to measure the iron abundance and compute the uncertainties based on the possible range of microturbulent velocities. Results. The O stars in the Magellanic Bridge have severely sub-SMC iron abundances that reach as low as 10.8% and 3.6% Fe⊙. The most Fe-deficient star also shows α-enhancement. These stars are the nearest extremely metal-poor O stars discovered to date. Conclusions. Our finding marks the first robust determination of O-star iron abundances in a metallicity regime comparable to dwarf galaxies such as Sextans A and Leo P. The iron abundances of the stars do not correlate with their oxygen abundances. Our results highlight the problem of using oxygen-based metallicities. The proximity of the stars in the Bridge combined with their different abundance patterns underlines that the interstellar medium of the Magellanic Bridge must be highly inhomogeneous and is not properly mixed.