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Non-LTE line formation for MgI/II: Abundances and stellar parameters

Model atom and first results on A-type stars

(Received 19 September 2000 / Accepted 11 January 2001)

Abstract
An extensive model atom for non-LTE line-formation calculations for neutral and singly-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 (A0 V), Leo (A0 Ib) and HD 92207 (A0 Iae) 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 for early A-type stars at all luminosities. Evidence is given that in late A and early F-type supergiants ( 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 0.3 dex -even in supergiants, but are essential for an accurate effective temperature determination. From the MgII spectrum, only the features at 4481 and 7877-96 Å 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.