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