  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 Angstroms.
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 < 1.2 eV
are excluded. This constrains the extent of any deviations from LTE that may
occur within this parameter range. While our result confirms non-LTE
calculations for some warm, metal-poor dwarfs, it may not be generalizable to
more metal-poor dwarfs, where deviations from LTE are predicted to be larger.
We also investigate trends of systematically lower abundances derived from Fe I
lines in the Balmer continuum region (3100-3700 Angstroms), and we conclude
that no proposed explanation for this effect can fully account for the
observations presently available.
