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Preprint I/99

The discovery of photospheric nickel in the hot DO white dwarf REJ0503-289

M.A. Barstow (1), S. Dreizler (2), J.B. Holberg (3), D.S. Finley (4), K. Werner (2), I. Hubeny (5) and E.M. Sion (6)

(1) Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
(4) Eureka Scientific Inc., 2452 Delmer St., Suite 100 Oakland, CA 94602
(5) Laboratory for Astronomy and Solar Physics, NASA/GSFC, Greenbelt, Maryland, MD 20711, USA
(6) Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085, USA

To appear in: Mon. Not. R. Astron. Soc.

Abstract. We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ0503-289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a He-rich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He=10^-6) implying a Fe/Ni ratio a factor 10 lower than seen in the H-rich objects (Ni/He=10^-5 for REJ0503-289). The abundance of nickel and various other elements heavier than He were determinde from GHRS spectra. we used two completely independent sets of NLTE model atmospheres which bot provide the same results. This not only reduces the possibility of systematic errors in our analysis but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining T_eff and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilising the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in T_eff and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of T_eff.

Key words: stars:abundances - stars:atmospheres - stars:white dwarfs - ultraviolet:stars

Paper (307k gzip'ed Postscript including figures)

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