Red Supergiant Stars as Cosmic Abundance Probes: II. NLTE Effects in J-band Silicon Lines

TL;DR

This paper investigates how non-LTE effects influence silicon line formation in J-band spectra of red supergiants, revealing significant abundance corrections crucial for accurate extragalactic chemical analysis.

Contribution

It provides detailed non-LTE calculations for silicon lines in red supergiants, extending previous work on iron and titanium, and discusses implications for stellar abundance studies.

Findings

Non-LTE effects cause significantly stronger silicon absorption lines.

Non-LTE silicon abundances are smaller than LTE estimates, with corrections up to -0.4 dex.

Effects are more pronounced at low metallicity and lower temperatures.

Abstract

Medium resolution J-band spectroscopy of individual red supergiant stars is a promising tool to investigate the chemical composition of the young stellar population in star forming galaxies. As a continuation of recent work on iron and titanium, detailed non-LTE calculations are presented to investigate the influence of NLTE on the formation of silicon lines in the J-band spectra of red supergiants. Substantial effects are found resulting in significantly stronger absorption lines of neutral silicon in non-LTE. As a consequence, silicon abundances determined in non-LTE are significantly smaller than in LTE with the non-LTE abundance corrections varying smoothly between -0.4 dex to -0.1 dex for effective temperatures between 3400K to 4400K. The effects are largest at low metallicity. The physical reasons behind the non-LTE effects and the consequences for extragalactic J-band abundance studies are discussed.