Consistent Iron Abundances Derived from Neutral and Singly Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-poor Stars

Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high-resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] ≈ −2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope . We locate archival UV spectra for three other warm dwarfs with [Fe/H] ≈ −3.3, −2.2, and −1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color–temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of Fe i and Fe ii lines with wavelengths from 2290 to 6430 Å. The [Fe/H] ratios derived separately from Fe i and Fe ii lines agree in all six stars, with [Fe ii /H]–[Fe i /H] ranging from +0.00 ± 0.07 to −0.12 ± 0.09 dex, when strong lines and Fe i lines with lower excitation potential