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W. L. Waldron (SM\&A Corp.)
The massive radiative driven stellar winds of O stars are very unstable, and combined with rapid rotation and expansion, are suspected to develop into highly structured plasma flows. Models suggest that these radiative instabilities develop into a distribution of stellar wind shocks, and produce the observed X-ray emission. Furthermore, large periodic propagating density enhancements, evident in UV wind lines, are believed to emanate from the stellar surface (i.e., the observed DAC phenomenon). Under these conditions, it is somewhat surprising that the observed X-ray emission seems to remain relatively constant. Although evidence supporting X-ray variability in a few O stars has emerged during the past several years, it is currently unclear as to whether these variable characteristics are common among all O stars. In addition, our analysis of archival X-ray data indicates that the observed X-ray emission appears to be dependent on the stellar rotation and the orientation of the rotational axis. This implies that the X-rays may be confined to the equatorial region of the stellar wind. We present models to illustrate how a random distribution of equatorial confined shocks can maintain an overall constancy in the X-ray emission, and provide periodic variability through the interaction of these shocks with propagating density enhancements.
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