Non-LTE line formation for Mg I/II: Abundances and stellar parameters - Model atom and first results on A-type stars

An extensive model atom for non-LTE line-formation calculations for neutral and single-ionized magnesium is presented, taking into account recent improvements in the atomic data. As a test and first application of the model, stellar parameters and magnesium abundances are determined on the basis of line-blanketed LTE model atmospheres for three stars: Vega (A0V), eta Leo (A0Ib) and HD 92207 (A0Iae) using high S/N and high resolution spectra at visual and near-IR wavelengths. The ionization equilibrium of MgI/II proves to be a sensible temperature indicator far early A-type stars at all luminosities. Evidence is given that in late A and early F-type supergiants (T-eff less than or similar to 8000 K) the determination of accurate stellar parameters is hampered by the presence of a pressure inversion region in the model atmospheres at line-formation depths. The MgI/II lines in the observations are reproduced simultaneously by the calculated line profiles with high accuracy. For Vega spectral synthesis in the UV region of the MgI/II resonance lines also proves excellent. consistency with the results from the visual. The dependence of the non-LTE effects on the atmospheric parameters is discussed with special emphasis on the supergiants where a strong radiation field at low particle densities favours deviations from LTE, especially in the minor ionic species of neutral magnesium. Non-LTE corrections turn out to be small in MgI - typically less than or similar to0.3 dex - even in supergiants, but are essential for an accurate effective temperature determination. From the MgII spectrum, only the features at lambda lambda 4481 and 7877-96 Angstrom react sensitively to non-LTE effects. Furthermore! the influence of microturbulence on the statistical-equilibrium calculations is investigated. The line strengths are found to be systematically affected.