Revisiting the iron abundance in the hyper iron-poor star HE$~$1327-2326 with UV COS/$HST$ data

TL;DR

This study refines the iron abundance measurement of the hyper metal-poor star HE 1327-2326 using UV and optical spectra, revealing minimal differences from previous estimates and emphasizing the importance of 3D NLTE effects in abundance analysis.

Contribution

First detection of Fe II lines in UV spectra of HE 1327-2326 and assessment of 3D NLTE effects on iron abundance measurements in hyper metal-poor stars.

Findings

Fe II lines detected in UV spectrum for the first time

Iron abundance determined as [Fe/H] = -5.20±0.12

Minimal difference from previous abundance estimates

Abstract

We present a new iron abundance analysis of the hyper metal-poor star HE$~$1327-2326, based on Fe II lines detected in its UV spectral range for the first time. In a Cosmic Origins Spectrograph (COS) spectrum, five new Fe II lines could be measured. A Si I line was also detected for the first time. We determine a 1D Local Thermodynamic Equilibrium (LTE) Fe II abundance of [Fe II/H] = -5.99$\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.