Analyses of light-ion spectra in stellar atmospheres. I. Magnesium II in B and O stars

A calculation of the spectrum of Mg ii in B and 0 stars, using a rather complete model atom allowing for many explicitly calculated transitions in steady-state statistical equilibrium, has been performed. These computations give a good fit to tbe observed equivalent widths of the A4481 line on the entire range from B5 through 06, assuming a solar abundance for magnesium. In contrast, for the earliest spectral types, the LTE predictions fit the data poorly, and are shown to yield abundances systematically in error by an order of magi'itude or more. These results open again serious questions concerning the validity of LTE abundances for at least the 0 and early B stars, and suggest strongly that further analyses are needed to delineate the ions and spectral types for which the LTE assumption fails.