Abstract
We present the observations of the photospheric X-ray spectra of hot DA white dwarfs, obtained with the 500 lines mm −1 Transmission Grating Spectrometer on EXOSAT. These spectra cover the full soft X-ray band, at high wavelength resolution and statistical quality. They allow us to do an accurate measurement of the photospheric parameters, particularly of effective temperature and chemical composition of the atmosphere.
We consider the case of HZ 43 in some detail. Model atmospheric spectra that satisfy all measured absolute optical, UV and X-ray fluxes turn out not to fit the shape of the measured X-ray spectrum. However, from a comparison of model spectra calculated with different model atmospheres codes we infer the existence of a 15% systematic uncertainty in the model fluxes at the shortest wavelengths (λ < 100 Å) in current model calculations. This can explain the fitting problem. Since the systematic uncertainty in the models is larger than the statistical uncertainty in the shape of the measured X-ray spectrum of HZ 43, we cannot at present use this measured shape to derive the effective temperature and gravity. We revert to broad band photometry, using the measured integrated soft X-ray flux and the optical flux, to determine T e = 45,000 – 54,000K, R/R ⊙ = 0.0140 – 0.0165. From the absence of the He II Ly edge at 227 Å in the measured spectrum, we set a upper limit on the photospheric helium abundance of He/H = 1.0 × 10−5; this upper limit is independent of the uncertainties in the model calculations mentioned above.
Preview
Unable to display preview. Download preview PDF.
Similar content being viewed by others
References
Dahn, C. C., et al. (1982), Astron. J., 87, 419.
de Korte, P. A. J., et al. (1981), Space Sci. Rev., 30, 495.
Heise, J., et al. (1988), Ap.J.,( in press).
Holberg, J. B., et al. (1980), Ap.J. (Letters), 242, L119.
Holberg, J. B., Wesemael, F., and Basile, J. (1986), Ap.J., 306, 629.
Paerels, F. B. S., et al. (1986a), Ap.J.(Letters), 309, L33.
Paerels, F. B. S., et al. (1986b), Ap.J., 308, 190.
Paerels, F. B. S., et al. (1988), Ap.J., 329, 849.
Petre, R., Shipman, H. L., and Canizares, C. (1986), Ap.J., 304, 356.
Taylor, B. (1985), Adv. Space Res., 5, 35.
Wesemael, F., et al. (1980), Ap.J.Suppl., 43, 159.
Author information
Authors and Affiliations
Editor information
Rights and permissions
Copyright information
© 1989 Springer-Verlag
About this paper
Cite this paper
Paerels, F., Heise, J. (1989). X-ray emission from hot DA white dwarfs; EXOSAT results, and implications for atmospheric models. In: Wegner, G. (eds) White Dwarfs. Lecture Notes in Physics, vol 328. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-51031-1_318
Download citation
DOI: https://doi.org/10.1007/3-540-51031-1_318
Published:
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-540-51031-4
Online ISBN: 978-3-540-46136-4
eBook Packages: Springer Book Archive