NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Résumé
Aims: {Aluminium is a key element to constrain the models of the chemical enrichment and the yields of the first supernovae. But obtaining precise Al abundances in extremely metal-poor (EMP) stars requires that the non-LTE effects be carefully taken into account. Methods: The NLTE profiles of the blue resonance aluminium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI applied to an atomic model of the Al atom with 78 levels of Al~I and 13 levels of Al~II, and compared to the observations. Results: With these new determinations, all the stars of the sample show a ratio Al/Fe close to the solar value: [Al/Fe] =-0.06 +- 0.10 with a very small scatter. These results are compared to the models of the chemical evolution of the halo using different models of SN~II and are compatible with recent computations. The sodium-rich giants are not found to be also aluminium-rich and thus, as expected, the convection in these giants only brings to the surface the products of the Ne-Na cycle.
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