Red Supergiant Stars as Cosmic Abundance Probes. III. NLTE effects in J-band Magnesium lines

TL;DR

This paper investigates non-LTE effects on magnesium lines in red supergiant stars' atmospheres, revealing significant impacts on spectral analysis crucial for studying stellar populations in galaxies.

Contribution

It provides detailed non-LTE calculations for Mg I lines in RSGs, improving the accuracy of chemical abundance determinations in extragalactic stellar studies.

Findings

Non-LTE effects strengthen Mg I lines significantly.

Non-LTE abundance corrections range from -0.4 to -0.1 dex.

Implications for chemical analysis of young stellar populations.

Abstract

Non-LTE calculations for Mg I in red supergiant stellar atmospheres are presented to investigate the importance of non-LTE for the formation of Mg I lines in the NIR J-band. Recent work using medium resolution spectroscopy of atomic lines in the J-band of individual red supergiant stars has demonstrated that technique is a very promising tool to investigate the chemical composition of the young stellar population in star forming galaxies. As in previous work, where non-LTE effects were studied for iron, titanium and silicon, substantial effects are found resulting in significantly stronger Mg I absorption lines. For the quantitative spectral analysis the non-LTE effects lead to magnesium abundances significantly smaller than in LTE with the non-LTE abundance corrections varying smoothly between -0.4 dex and -0.1 dex for effective temperatures between 3400 K and 4400 K. We discuss the physical reasons of the non-LTE effects and the consequences for extragalactic J-band abundance studies using individual red supergiants in the young massive galactic double cluster h and chi Persei.