Revisiting the Iron Abundance in the Hyper Iron-poor Star HE 1327-2326 with UV COS/HST Data

Abstract

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 ± 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 ± 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 across the entire UV-optical wavelength range. An iron abundance of {{[Fe/H]}} = -5.20 ± 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS 5-26555. These observations are associated with program GO-14151.