A possible origin for X-rays from O stars.

X-ray spectra of hot, massive stars provide convincing evidence for thermal emission that extends far out into their stellar winds. Accordingly, strong shocks were proposed as sources of the X-ray emission, where the shocks result from the line-driven instability. We show from hydrodynamic simulations that the emission from individual shocks which grow out of initially small perturbations may fall one or two orders of magnitude below the observed flux. Instead, we find that mutual collisions of dense shells of gas formed in deep wind regions can lead to shocks with a much stronger emission which almost matches the observed flux. This model predicts strong variability of the X-ray emission, which is not observed. We propose that - in contrast to the presently assumed spherical symmetry of the wind - the emission stems from a large number of independent, radial cones so that fluctuations average out over the whole emitting volume.