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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint A/05

Deposing the Cool Corona of KPD0005+5106

Jeremy J. Drake (1), Klaus Werner (2)

(1) Smithsonian Astrophysical Observatory, MS-3, 60 Garden Street, Cambridge, MA 02138
(2) Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany

To be published in: Astrophysical Journal

Abstract. The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf KPD0005+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 Å 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.
Key words: stars: activity - stars: coronae - stars: white dwarfs - stars: mass loss - X-rays: stars

Preprint (427 kb PDF file including figures)

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