Red Supergiant Stars as Cosmic Abundance Probes: NLTE Effects in J-band Iron and Titanium Lines

TL;DR

This study performs detailed non-LTE calculations for red supergiant stars to assess how NLTE effects influence iron and titanium line formation in the J-band, supporting accurate cosmic abundance measurements.

Contribution

It provides the first comprehensive non-LTE analysis of iron and titanium lines in RSGs' J-band spectra, refining abundance correction estimates for extragalactic studies.

Findings

Non-LTE corrections for iron are under 0.05 dex for temperatures 3400K-4200K.

Titanium non-LTE corrections range from -0.4 to +0.2 dex depending on temperature.

Results support previous LTE-based metallicity determinations with improved accuracy.

Abstract

Detailed non-LTE calculations for red supergiant stars are presented to investigate the influence of NLTE on the formation of atomic iron and titanium lines in the J-band. With their enormous brightness at J-band red supergiant stars are ideal probes of cosmic abundances. Recent LTE studies have found that metallicities accurate to 0.15 dex can be determined from medium resolution spectroscopy of individual red supergiants in galaxies as distant as 10 Mpc. The non-LTE results obtained in this investigation support these findings. Non-LTE abundance corrections for iron are smaller than 0.05 dex for effective temperatures between 3400K to 4200K and 0.1 dex at 4400K. For titanium the non-LTE abundance corrections vary smoothly between -0.4 dex and +0.2 dex as a function of effective temperature. For both elements, the corrections also depend on stellar gravity and metallicity. The physical reasons behind the non-LTE corrections and the consequences for extragalactic J-band abundance studies are discussed.