Deposing the Cool Corona of KPD 0005+5106

Abstract

The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner & Barstow (1993). It defied the best non-LTE modelling attempts in terms of photospheric emission from He-dominated atmospheres including C, N and O and was instead interpreted as the first evidence for a coronal plasma around a white dwarf. We show here that a recent high resolution Chandra LETGS spectrum has more structure than expected from a thermal bremsstrahlung continuum and lacks the narrow lines of H-like and He-like C expected from a coronal plasma. Moreover, a coronal model requires a total luminosity more than two orders of magnitude larger than that of the star itself. Instead, the observed 20-80 AA flux is consistent with photospheric models containing trace amounts of heavier elements such as Fe. The soft X-ray flux is highly sensitive to the adopted metal abundance and provides a metal abundance diagnostic. The weak X-ray emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from the photosphere and requires alternative explanations. We echo earlier speculation that such emission arises in a shocked wind. Despite the presence of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be trapped by resonant scattering within the emitting plasma and destroyed by photoelectric absorption.

arXiv comment15 Pages, 4 figures. Accepted for the Astrophysical Journal

References in corpus (8)

• Ionization balance for optically thin plasmas: rate coefficients for all atoms and ions of the elements H to Ni
• CHIANTI - an Atomic Database for Emission Lines. Paper VI: Proton Rates and Other Improvements
• Chandra and FUSE spectroscopy of the hot bare stellar core H1504+65
• Hard X-ray Emission Associated with White Dwarfs
• Iron abundance in hot hydrogen-deficient central stars and white dwarfs from FUSE, HST, and IUE spectroscopy
• Equilibrium abundances in hot DA white dwarfs as derived from self-consistent diffusion models I. Analysis of spectroscopic EUVE data
• The Nebular Environment and Enigmatic Hard X-ray Emission of the Hot DO White Dwarf KPD 0005+5106
• Discovery of an OVI Emitting Nebula around the Hot White Dwarf KPD 0005+5106

Cited by in corpus (5)

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• Listening for Dark Photon Radio from the Galactic Centre
• Hard X-ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Sub-stellar Companions
• Chandra grating spectroscopy of three hot white dwarfs
• A New Analysis of the O VI Emitting Nebula around KPD 0005+5106