The High-Resolution Soft X-Ray Spectrum of HZ 43

Abstract

The authors present the high-resolution soft X-ray spectrum in the range 50 - 350 Å of the hot DA white dwarf HZ 43, obtained with the 500 lines mm^-1 grating spectrometer on EXOSAT. Most remarkably, the spectrum indicates that the photosphere of HZ 43 consists solely of hydrogen, to a high degree of purity. Using a grid of hot, high-gravity LTE model atmospheres, the authors derive accurate atmospheric parameters for HZ 43 as well as constraints on the radius and luminosity of the white dwarf and on the column density of absorbing interstellar material. The effective temperature is T_eff = 59,000 - 62,000K, and the fractional number density of helium in the photosphere is n(He)/n(H) ≤ 6×10^-6.