Detection of Al II in the Ultraviolet Spectra of Metal-Poor Stars: An
  Empirical LTE Test of NLTE Aluminum Abundance Calculations
  We report the detection of an Al II line at 2669.155 Angstroms in 11
metal-poor stars, using ultraviolet spectra obtained with the Space Telescope
Imaging Spectrograph on board the Hubble Space Telescope. We derive Al
abundances from this line using a standard abundance analysis, assuming local
thermodynamic equilibrium (LTE). The mean [Al/Fe] ratio is -0.06 +/- 0.04
(sigma = 0.22) for these 11 stars spanning -3.9 < [Fe/H] < -1.3, or [Al/Fe] =
-0.10 +/- 0.04 (sigma = 0.18) for 9 stars spanning -3.0 < [Fe/H] < -1.3 if two
carbon-enhanced stars are excluded. We use these abundances to perform an
empirical test of non-LTE (NLTE) abundance corrections predicted for resonance
lines of Al I, including the commonly-used optical Al I line at 3961 Angstroms.
The Al II line is formed in LTE, and the abundance derived from this line
matches that derived from high-excitation Al I lines predicted to have minimal
NLTE corrections. The differences between the abundance derived from the Al II
line and the LTE abundance derived from Al I resonance lines are +0.4 to +0.9
dex, which match the predicted NLTE corrections for the Al I resonance lines.
We conclude that the NLTE abundance calculations are approximately correct and
should be applied to LTE abundances derived from Al I lines.
