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 Å 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 (σ = 0.22) for these 11 stars spanning − 3.9 < [Fe/H] < −1.3, or [Al/Fe] = −0.10 ± 0.04 (σ = 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 Å. 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.