NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars

TL;DR

This study accurately determines aluminium abundances in extremely metal-poor stars using non-LTE modeling, revealing a near-solar Al/Fe ratio and providing insights into early chemical enrichment.

Contribution

It introduces a detailed non-LTE analysis of aluminium lines in EMP stars, improving abundance accuracy and comparing results with chemical evolution models.

Findings

Al/Fe ratio close to solar with minimal scatter

Results align with recent supernova nucleosynthesis models

No correlation between sodium-rich giants and aluminium abundance

Abstract

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.