2006ApJ...648..565O

We investigate the degree to which the nearly symmetric form of X-ray emission lines seen in Chandra spectra of early-type supergiant stars could be explained by the possibly porous nature of their spatially structured stellar winds. Such porosity could effectively reduce the bound-free absorption of X-rays emitted by embedded wind shocks, and thus allow a more similar transmission of redshifted and blueshifted emission from the back and front hemispheres, respectively. To obtain the localized self-shielding that is central to this porosity effect, it is necessary that the individual clumps be optically thick. In a medium consisting of clumps of size ℓ and volume filling factor f, we argue that the general modification in effective opacity should scale approximately as κ_eff~κ/1+τ_c, where, for a given atomic opacity κ and mean density ρ, the clump optical thickness scales as τ_c=κρℓ/f. For a simple wind structure parameterization in which the ``porosity length'' h≡ℓ/f increases with local radius r as h=h'r, we find that a substantial reduction in wind absorption requires a quite large porosity scale factor, h'≳1, implying large porosity lengths h≳r. The associated wind structure must thus have either a relatively large scale ℓ≲r, or a small volume filling factor f~ℓ/r≪1, or some combination of these. We argue that the relatively small-scale, moderate compressions generated by intrinsic instabilities in line driving are unlikely to give such large porosity lengths. This raises questions about whether porosity effects could play a significant role in explaining nearly symmetric X-ray line profiles, leaving the prospect of instead having to invoke a substantial (approximately a factor of 5) downward revision in the assumed mass-loss rates.