Revisiting the Iron Abundance in the Hyper Iron-poor Star HE 1327–2326 with UV COS/HAT 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 = -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.