  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.
