Iron in Hot DA White Dwarfs

Abstract

We present a study of the iron abundance pattern in hot, hydrogen-rich (DA) white dwarfs. The study is based on new and archival far-ultraviolet spectroscopy of a sample of white dwarfs in the temperature range 30,000 K<~T_eff<~64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic Explorer, along with spectra obtained with the Hubble Space Telescope Imaging Spectrograph and the International Ultraviolet Explorer, sample Fe III-Fe VI absorption lines, enabling a detailed iron abundance analysis over a wider range of effective temperatures than previously afforded. The measurements reveal abundance variations in excess of 2 orders of magnitude between the highest and the lowest temperatures probed, but also show considerable variations (over 1 order of magnitude) between objects with similar temperatures and surface gravities. Such variations in cooler objects may be imputed to accretion from unseen companions or so-called circumstellar debris, although the effect of residual mass loss and selective radiation pressure in the hottest objects in the sample remain dominant.