Non‐LTE Line‐blanketed Model Atmospheres of Hot Stars. III. Hot Subdwarfs: The sdO Star BD +75o325

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Abstract
We have made a detailed comparison of results of spectroscopic analysis using three differents types of model atmospheres: classical non-LTE H-He models; approximate non-LTE line-blanketed models (with only a subset of Fe and Ni lines: those originating from transitions between levels with measured energies); and non-LTE fully blanketed models. The three models were applied to the sdO star BD +75°325, adopted as a test case. We demonstrate that the effects of line blanketing are very important: the best fit of the observed H and He lines is achieved for Teff = 58,000 K when using H-He models, Teff = 55,000 K for approximate non-LTE line-blanketed models, and Teff = 52,000 K for fully blanketed non-LTE models. Using the high-resolution Goddard High Resolution Spectrograph spectrum of BD +75°325 and our final fully blanketed model, we have derived reliable abundances of He, C, N, O, Si, Fe, and Ni. We find that BD +75°325 is an He-enriched star (He/H = 1 by number), whose surface exhibits CNO-cycle products, i.e., N-rich (AN = 1.5 × 10-3 by number, or 4.2 × 10-3 by mass fraction), and C and O deficient (by factor of about 100 with respect to the solar value). We also find a significant surface depletion of silicon and an enhancement of iron and nickel. We argue that these anomalous abundances reflect some mixing with processed material from the core, with subsequent modification at the surface by diffusion processes. Finally, BD +75°325 possesses a weak wind. Using a simplified description of the wind, we have derived a preliminary value of the mass loss rate: =1.5 × 10−11 M☉ yr-1.