NLTE Line Formation for Mg i and Mg ii in the Atmospheres of B–A–F–G–K Stars

Non-local thermodynamical equilibrium (NLTE) line formation for Mg i and Mg ii lines is considered in classical 1D LTE model atmospheres of the Sun and 17 stars with reliable atmospheric parameters and in a broad range of spectral types: 3900 K ≤ ≤ 17,500 K, 1.1 ≤ log g ≤ 4.7, and −2.6 ≤ [Fe/H] ≤ +0.4. We find that, for each star, NLTE leads to smaller line-to-line scatter. For 10 stars, NLTE leads to consistent abundances of Mg i and Mg ii, while the difference in LTE abundance varies between −0.21 and +0.23 dex. We obtain an abundance discrepancy between Mg i and Mg ii in two very metal-poor stars, HD 140283 and HD 84937. The origin of these abundance differences remains unclear. Our standard NLTE modeling predicts Mg i emission lines at 7.736, 11.789, 12.224, and 12.321 μm in the atmospheres with ≤ 7000 K. We reproduce well the Mg i 12.2 and 12.3 μm emission lines in Procyon. However, for the Sun and three K-giants, the predicted Mg i emission lines are too weak compared with the observations. For stars with 7000 K ≤ ≤ 17,500 K, we recommend the Mg ii 3848, 3850, 4384, 4390, 4427, and 4433 Å lines for Mg abundance determinations even at the LTE assumption due to their small NLTE effects. The Mg i 4167, 4571, 4702, 5528, 5167, 5172, and 5183 Å lines can be safely used in the LTE analysis of stars with 7000 K < ≤ 8000 K. For the hotter stars, with from 8000 K to 9500 K, the NLTE effects are minor only for Mg i 4167, 4702, and 4528 Å.