Stellar Wind Sources

The detection of X-ray emission from hot stars was one of the first science results obtained from observations with the Einstein Observatory. Systematic studies of hot main-sequence stars based on the extensive RASS data have shown that stars with spectral type earlier than B3 emit a fraction of ~10-7 of their bolometric luminosity in the X-ray band (see Fig. 10.20). Stars of later spectral type show significantly larger Lx/Lbol ratios as well as a larger spread of X-ray luminosities, suggesting a qualitative difference in the underlying emission mechanism.

Fig. 10.20 X-ray luminosity vs. bolometric luminosity for a flux-limited sample of OB stars observed during the RASS. Solid lines represent regression fits for Lbol < 1038 ergs-1 and Lbol > 1038 ergs-1, and the dashed line shows the canonical relation for hot star X-ray emission of log (Lx/Lboj) = -7. Typical ranges for some classes of late-type stars are indicated by the bars on the left (Fig. 4 from [4])

Fig. 10.20 X-ray luminosity vs. bolometric luminosity for a flux-limited sample of OB stars observed during the RASS. Solid lines represent regression fits for Lbol < 1038 ergs-1 and Lbol > 1038 ergs-1, and the dashed line shows the canonical relation for hot star X-ray emission of log (Lx/Lboj) = -7. Typical ranges for some classes of late-type stars are indicated by the bars on the left (Fig. 4 from [4])

Spectral analysis even of the low spectral resolution Einstein Observatory and ROSAT data show that OB stars produce relatively soft (kT ^0.5 keV) X-ray emission, which is significantly less variable than the X-ray flux observed from low-mass stars. The observed X-ray emission of early-type stars is widely attributed to a myriad of small shocks, expected to form due to instabilities in the radiatively-driven winds of these hot stars [28]. To arrive at X-ray producing temperatures, the shock velocities must reach a few hundreds kilometers per second, a value easily achieved in O star winds, where typical wind terminal velocities of up to 3000 km s-1 are found. Integrating over a multitude of small shocks yields an approximately constant X-ray luminosity, thus explaining the low level of observed variability.

The development of a radiatively accelerated wind depends both on a star's radiation field indicated by its spectral type (or Teff) and its gravity. For low temperatures (e.g., Teff < 10000 K), only weak-gravity stars can have winds, whereas on the main-sequence, for high temperatures (Teff > 20,000 K), winds can exist even for high-gravity stars. In practice, the transition between the weak-wind and fastwind regimes near the main-sequence takes place for Teff ~ 14,000 - 18,000 K, i.e., between spectral types B3 and B5, in rough agreement with the observed transition in the Lx/Lbol diagram.

An important question that could not be addressed with the Einstein Observatory and ROSAT data is the production site of the X-ray emission. With terminal velocities of O star winds extending up to 3 000 km s-1, one expects line broadening depending on the location of the X-ray emission site within the wind. Such line broadening can be resolved with the spectrometers onboard XMM-Newton and Chandra, but a discussion of these new results would go beyond the scope of this chapter.

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