Exospheric models for the X-ray emission from single Wolf-Rayet stars

We review existing ROSAT detections of single Galactic Wolf-Rayet (WR) star s and develop wind models to interpret the X-ray emission. The ROSAT data, consisting of bandpass detections from the ROSAT All-Sky Survey (RASS) and some pointed observations, exhibit no correlations of the WR X-ray luminosi ty (L-X) with any star or wind parameters of interest (e.g. bolometric lumi nosity, mass-loss rate or wind kinetic energy), although the dispersion in the measurements is quite large. The lack of correlation between X-ray lumi nosity and wind parameters among the WR stars is unlike that of their proge nitors, the O stars, which show trends with such parameters. In this paper we seek to (i) test by how much the X-ray properties of the WR stars differ from the O stars and (ii) place limits on the temperature T-X and filling factor f(X) of the X-ray-emitting gas in the WR winds. Adopting empirically derived relationships for T-X and f(X) from O-star winds, the predicted X- ray emission from WR stars is much smaller than observed with ROSAT. Abando ning the T-X relation from O stars, we maximize the cooling from a single-t emperature hot gas to derive lower limits for the filling factors in WR win ds. Although these filling factors are consistently found to be an order of magnitude greater than those for O stars, we find that the data are consis tent (albeit the data are noisy) with a trend of i(X) proportional to ((M) over dot/upsilon(infinity))(-1) in WR stars, as is also the case for O star s.