ORFEUS II echelle spectra: On the influence of iron-group line blanketing in the Far-UV spectral range of hot subdwarfs

Abstract

We present an analysis of the subdwarf O star Feige 67 with a fully metal-line blanketed NLTE model atmosphere based on high-resolution Far-UV (912-2000 A) and Near-UV (2000-3400 A) spectra from new ORFEUS II echelle observations and the IUE final archive. The Far-UV spectra are heavily blanketed by iron and nickel lines, preventing the detection of the stellar continuum and complicating the abundance analysis. Important points concerning the account for blanketing by millions of iron-group lines and for an accurate determination of iron and nickel abundances are discussed: The usage of all theoretically and experimentally known line opacities of the iron-group elements, the consideration of a broad wavelength range for a reliable determination of the stellar continuum flux, and the role of interstellar reddening. This paper outlines a basic approach for spectral analysis of future high-resolution Far-UV spectra of hot compact stars. During this study we re-analyzed the iron and nickel abundance of our exemplary object Feige 67 and confirm their extraordinarily amount.