Iron abundances from optical Fe III absorption lines in B-type stellar spectra

TL;DR

This study evaluates the effectiveness of optical Fe III absorption lines as diagnostics for iron abundance in B-type stars, comparing different model atmospheres and confirming LTE estimates align with Galactic expectations.

Contribution

It identifies specific Fe III transitions suitable for iron abundance estimation in early B-type stellar spectra, demonstrating the reliability of optical lines over UV lines.

Findings

Non-LTE effects are minimal in Fe III lines.

Optical Fe III lines provide consistent abundance estimates.

Recommended transitions for future studies are listed.

Abstract

The role of optical Fe III absorption lines in B-type stars as iron abundance diagnostics is considered. To date, ultraviolet Fe lines have been widely used in B-type stars, although line blending can severely hinder their diagnostic power. Using optical spectra, covering a wavelength range ~ 3560 - 9200 A, a sample of Galactic B-type main-sequence and supergiant stars of spectral types B0.5 to B7 are investigated. A comparison of the observed Fe III spectra of supergiants, and those predicted from the model atmosphere codes TLUSTY (plane-parallel, non-LTE), with spectra generated using SYNSPEC (LTE), and CMFGEN (spherical, non-LTE), reveal that non-LTE effects appear small. In addition, a sample of main-sequence and supergiant objects, observed with FEROS, reveal LTE abundance estimates consistent with the Galactic environment and previous optical studies. Based on the present study, we list a number of Fe III transitions which we recommend for estimating the iron abundance from early B-type stellar spectra.