ALUMINUM IN METAL-POOR STARS

Previous calculations of the statistical equilibrium of aluminium in t he 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 on NLTE populations lead to significantl y higher Al abundances than those calculated from LTE. For stars of in termediate metallicity between -1.0 < [Fe/H] < -0.5 some overabundance relative to iron is found. For more metal-poor stars the overabundanc e disappears and approaches the solar ratio, [AL/Fe] = 0. Only a weak overabundance in the [Al/Mg] ratio is detected for stars with intermed iate metallicity and a small underabundance of -0.2 to -0.3 dex for th e metal-poor stars. From investigation of both solar and stellar Al sp ectra the influence of hydrogen collisions could be better estimated. The previously defined atomic model thus had to be slightly modified t o fit both metal-rich and metal-poor stars. Compared with LTE analyses the present results completely change the chemical enrichment scenari o with [Al/Fe] now following the trend of primary elements for all met al-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 ex plained in terms of stellar nucleosynthesis because it could result fr om our reference to LTE abundances for Mg and Fe.