Non-LTE effects in MgI lines for various types of stars

We have performed a detailed statistical-equilibrium analysis based on a 49 -level model of the magnesium atom for the atmospheres of stars of various spectral types: T-eff = 4500-12000 K, log g = 0.0-4.5, and [M/H] = 0 to -3. In the atmospheres of stars with T-eff > 5500 K, deviations from LTE for M g I are due to photoionization by ultraviolet radiation from the 3p level; i.e., neutral magnesium is in a state of "superionization." When T-eff < 55 00 K, the populations of the Mg I levels differ from their LTE values due t o radiative processes in bound-bound transitions. We analyzed Mg I lines in the solar spectrum in order to empirically refine certain atomic parameter s (the van der Waals broadening constant C-6 and cross sections for photoio nization and collisional interactions with hydrogen atoms) and the magnesiu m abundance in the solar atmosphere. We studied non-LTE effects for five Mg I lines for a wide range of stellar parameters. In the case of dwarfs and subdwarfs, the magnitude of non-LTE corrections to magnesium abundances doe s not exceed 0.1 dex for the lambda lambda 4571, 4703, 5528, and 5711 Angst rom lines but can be as large as +/-0.2 dex for the lambda lambda 3829-3838 , 5172, and 5183 Angstrom lines. The non-LTE corrections for giants and sup ergiants do not exceed 0.15 dex for the lambda lambda 4571 and 5711 Angstro m lines but can reach +/-0.20 dex and even more for the lambda lambda 4703, 5528, 3829-3838, 5172, and 5183 Angstrom lines. (C) 2000 MAIK "Nauka/Inter periodica".