Far-Ultraviolet Spectra of Main-Sequence O Stars at Extremely Low
  Metallicity
  Metal-poor massive stars dominate the light we observe from star-forming
dwarf galaxies and may have produced the bulk of energetic photons that
reionized the universe at high redshift. Yet, the rarity of observations of
individual O stars below the $20\%$ solar metallicity ($Z_\odot$) of the Small
Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of
metal-poor stellar populations. We present new Hubble Space Telescope
far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P
($3\%\,Z_\odot$), Sextans A ($6\%\,Z_\odot$), and WLM ($14\%\,Z_\odot$). We
quantify equivalent widths of photospheric metal lines and strengths of
wind-sensitive features, confirming that both correlate with metallicity. We
infer the stars' fundamental properties by modeling their FUV through
near-infrared spectral energy distributions and identify stars in the SMC with
similar properties to each of our targets. Comparing to the FUV spectra of the
SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and
(2) the most metal-poor star in Leo P is driving a much weaker stellar wind
than its SMC counterparts. We measure projected rotation speeds and find that
the two most metal-poor stars have high $v
\,\mathrm{sin}(i)\,\geq\,290\,\mathrm{km}\,\mathrm{s}^{-1}$, and estimate just
a $3-6\%$ probability of finding two fast rotators if the metal-poor stars are
drawn from the same $v \,\mathrm{sin}(i)$ distribution observed for O dwarfs in
the SMC. These observations suggest that models should include the impact of
rotation and weak winds on ionizing flux to accurately interpret observations
of metal-poor galaxies in both the near and distant universe.
