Non-LTE aluminium abundances in late-type stars

Abstract

Aims. Aluminium plays a key role in studies of the chemical enrichment of the Galaxy and of globular clusters. However, strong deviations from LTE (non-LTE) are known to significantly affect the inferred abundances in giant and metal-poor stars. Methods. We present NLTE modeling of aluminium using recent and accurate atomic data, in particular utilizing new transition rates for collisions with hydrogen atoms, without the need for any astrophysically calibrated parameters. For the first time, we perform 3D NLTE modeling of aluminium lines in the solar spectrum. We also compute and make available extensive grids of abundance corrections for lines in the optical and near-infrared using one-dimensional model atmospheres, and apply grids of precomputed departure coefficients to direct line synthesis for a set of benchmark stars with accurately known stellar parameters. Results. Our 3D NLTE modeling of the solar spectrum reproduces observed center-to-limb variations in the solar spectrum of the 7835 Å line as well as the mid-infrared photospheric emission line at 12.33 µm. We infer a 3D NLTE solar photospheric abundance of A(Al) = 6.43 ± 0.03, in exact agreement with the meteoritic abundance. We find that abundance corrections vary rapidly with stellar parameters; for the 3961 Å resonance line, corrections are positive and may be as large as +1 dex, while corrections for subordinate lines generally have positive sign for warm stars but negative for cool stars. Our modeling reproduces the observed line profiles of benchmark K-giants, and we find abundance corrections as large as −0.3 dex for Arcturus. Our analyses of four metal-poor benchmark stars yield consistent abundances between the 3961 Å resonance line and lines in the UV, optical and near-infrared regions. Finally, we discuss implications for the galactic chemical evolution of aluminium.

On the AGB stars of M 4: a robust disagreement between spectroscopic observations and theory

“…All absorption lines measured are known to suffer from non-LTE effects (Bergemann & Nordlander 2014). Abundances of O, Na, and Al were corrected for these non-LTE effects by interpolation of the grids from Amarsi et al (2016a, O), Lind et al (2011, Na), and Nordlander & Lind (2017, Al). Mg was not corrected for non-LTE because it is known (and confirmed in this study) to be homogeneous in M 4.…”

Accuracy and Precision of Industrial Stellar Abundances

“…In APOGEE, three Al i lines are present, although one seems to be more robust for different types of stars. Non-LTE studies of optical and IR lines have been performed by Nordlander & Lind (2017). • Silicon Among the 45 Si i lines visible in GES spectra, only three have good atomic data and are blend-free in dwarfs and giants.…”

High-precision stellar abundances of the elements: methods and applications

“…However, the situation may be more problematic for other chemical elements. One example is aluminium which has recently been studied with 3D models in non-LTE for a 42 level atom by Nordlander and Lind (2017). While the differences between 3D non-LTE results and 1D LTE results are small for the Sun as well as for the dwarf HD 22879 with solar temperature but [Fe/H] = −0.86, and in fact also for the Pop I K0 giant Pollux, they increase to 0.4 dex for the metal poor stars HD 140283 and HD 84937.…”