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 analyzed by Fleming, Werner, & Barstow. It defied the best non-LTE modeling 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 2 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 and coworkers 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 and 10, no X-ray O VIII Lyα 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2005
- DOI:
- 10.1086/429661
- arXiv:
- arXiv:astro-ph/0502051
- Bibcode:
- 2005ApJ...625..973D
- Keywords:
-
- Stars: Activity;
- Stars: Coronae;
- Stars: Mass Loss;
- Stars: White Dwarfs;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 15 Pages, 4 figures. Accepted for the Astrophysical Journal