Revisiting the iron abundance in the hyper iron-poor star HE$~$1327-2326
  with UV COS/$HST$ data
  We present a new iron abundance analysis of the hyper metal-poor star
HE$~$1327-2326, based on Fe II lines detected in its UV spectral range for the
first time. In a Cosmic Origins Spectrograph (COS) spectrum, five new Fe II
lines could be measured. A Si I line was also detected for the first time. We
determine a 1D Local Thermodynamic Equilibrium (LTE) Fe II abundance of [Fe
II/H] = -5.99$\pm$0.25. We also investigate departures from LTE for both Fe I
and Fe II lines. Guided by 3D Non-LTE (NLTE) analyses of other well-studied
metal-poor stars, we identify potential "residual" 3D effects in HE$~$1327-2326
arising from the absence of full 3D NLTE Fe calculations. Accordingly, we
employ measurements of 10 weak Fe I lines previously detected in an optical
spectrum of HE$~$1327-2326, as no Fe I lines are detectable in our UV spectrum.
Following our previous work, we adopt the 1D NLTE Fe I abundance of [Fe/H] =
-5.20$\pm$0.12 for HE$~$1327-2326. Adopting a value based on the optical Fe I
rather than UV lines was heavily informed by our extensive investigation of
model atmosphere and radiative transfer effects on different lines accross the
entire UV-optical wavelength range. An iron abundance of [Fe/H] =
-5.20$\pm$0.12 is only 0.2 dex higher than what was used in previous studies.
Accordingly, no previous conclusions regarding the nature of the star are
affected.
