Iron and nickel abundances of subluminous O-stars. I. NLTE-model atmospheres with line blanketing by iron group elements.

Abstract

NLTE model atmospheres for hot evolved stars including line blanketing of all iron group elements were first constructed by Dreizler & Werner (1993). In this paper we improve computational details and check the adopted simplifications. We also present a systematic investigation of the consequences of iron group line blanketing on the atmospheric structure (temperature stratification and ionization equilibria of iron and nickel) and for various observational quantities in the case of hot sdO model atmospheres. The inclusion of a vast number of iron group lines yields a substantial flux blocking in the vicinity of the He{2} 228A edge. The resulting effects on the optical line profiles of hydrogen and helium lines are none the less marginal. The most pronounced differences occur in the cores of these lines, in particular of the He{2} 4686A line. The new models are applied to an abundance analysis of Fe{5}, Fe{6} and Ni{6} lines in the high resolution IUE spectrum of the hot sdO star BD+28 4211. An underabundance of iron by 1dex with respect to the sun is found, while nickel displays a solar abundance. The reduced n_Fe_/n_Ni_ ratio (~1dex compared to the solar value) is an important hint that radiative levitation may hinder the gravitational settling of these species. It is demonstrated that the Fe{5}/Fe{6} ionization equilibrium is a reliable indicator for T_eff_ consistent with results from the optical He{1}/He{2} ionization equilibrium routinely used for spectroscopic analyses.