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Astron. Astrophys. 320, 899-912 (1997)

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The X-ray emission from shock cooling zones in O star winds *

A. Feldmeier 1, R.-P. Kudritzki 1, 2, R. Palsa 1, A.W.A. Pauldrach 1 and J. Puls 1

1 Institut für Astronomie und Astrophysik der Universität, Scheinerstr. 1, D-81679 München, Germany
2 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, Germany

Received 16 April 1996 / Accepted 17 September 1996

Abstract

A semi-empirical model is developed for the X-ray emission from O star winds, and used to analyze recent ROSAT PSPC spectra. The X-rays are assumed to originate from cooling zones behind shock fronts, where the cooling is primarily radiative at small radii in the wind, and due to expansion at large radii. The shocks are dispersed in a cold background wind whose X-ray opacity is provided by detailed NLTE calculations.

This model is a natural extension of the Hillier et al. (1993) model of isothermal wind shocks. By assuming spatially constant shock temperatures, these authors achieved good fits to the data only by postulating two intermixed shock families of independent temperature and filling factor - i.e., by adjusting in parallel four parameters. By applying the present model to the analysis of high S/N PSPC spectra of three O-stars ([FORMULA]  Pup, [FORMULA]  Ori, [FORMULA]  Ori), we achieve fits of almost the same quality with only two parameters. This supports the idea that the two- or multi-component X-ray spectra are indeed due to stratified cooling layers.

Key words: hydrodynamics – radiative transfer – shock waves – stars: early-type – X-rays: stars

* Based on observations obtained with the ROSAT X-ray satellite.

Send offprint requests to: A. Feldmeier: feld@usm.uni-muenchen.de

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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