On the sensitivity of He I singlet lines to the Fe IV model atom in O stars

¹ Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas, CSIC, Serrano 121, 28006 Madrid, Spain e-mail: najarro@damir.iem.csic.es
² Dept. of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara St., Pittsburgh, PA 15260, USA
³ Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 Munich, Germany
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁵ Max Planck Institüt für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Received: 7 November 2005
Accepted: 27 April 2006

Abstract

Aims.Recent calculations and analyses of O star spectra have revealed discrepancies between theory and observations, and between different theoretical calculations, for the strength of optical  singlet transitions. We investigate the source of these discrepancies.

Methods.Using a non-LTE radiative transfer code we have undertaken detailed test calculations for a range of O star properties. Our principal test model has parameters similar to those of the O9V star, 10 Lac.

Results.We show that the discrepancies arise from uncertainties in the radiation field in the 1s^2 1S–1s 2p ¹P° transition near 584 Å. The radiation field at 584 Å is influenced by model assumptions, such as the treatment of line-blanketing and the adopted turbulent velocity, and by the  atomic data. It is shown that two  transitions near 584 Å can have a substantial influence on the strength of the  singlet transitions.

Conclusions.Because of the difficulty of modeling the  singlet lines, particularly in stars with solar metalicity, the  triplet lines should be preferred in spectral analyses. These lines are much less sensitive to model assumptions.