Aluminium in metal-poor stars

Previous calculations of the statistical equilibrium of aluminium in the solar photosphere have shown that NLTE populations hardly affect Al line formation in the Sun; however, in metal-poor stars the influence of electron collisions is reduced, and a UV radiation field enhanced due to smaller background line opacity results in more pronounced NLTE effects. Thus analyses based onNLTEpopulations lead to significantly higher Al abundances than those calculated from LTE. For stars of intermediate metallicity between −1.0 < [Fe/H] < −0.5 some overabundance relative to iron is found. For more metal-poor stars the overabundance disappears and approaches the solar ratio, [Al/Fe] = 0. Only a weak overabundance in the [Al/Mg] ratio is detected for stars with intermediate metallicity and a small underabundance of −0.2 to −0.3 dex for the metal-poor stars. From investigation of both solar and stellar Al spectra the influence of hydrogen collisions could be better estimated. The previously defined atomic model thus had to be slightly modified to fit both metal-rich and metal-poor stars. Compared with LTE analyses the present results completely change the chemical enrichment scenario with [Al/Fe] now following the trend of primary elements for all metal-poor stars. The hump of enhanced Al/Fe values for stars between −1.0 < [Fe/H] < −0.5 does not seem to be an artefact. It nearly vanishes for the [Mg/Fe] abundance ratios. It may not necessarily have to be explained in terms of stellar nucleosynthesis because it could result from our reference to LTE abundances for Mg and Fe.