The Effect of Porosity on X‐Ray Emission‐Line Profiles from Hot‐Star Winds

We investigate the degree to which the nearly symmetric form of X-rayemission lines seen in Chandra spectra of early-type supergiant stars could beexplained by a possibly porous nature of their spatially structured stellarwinds. Such porosity could effectively reduce the bound-free absorption ofX-rays emitted by embedded wind shocks, and thus allow a more similartransmission of red- vs. blue-shifted emission from the back vs. fronthemispheres. For a medium consisting of clumps of size l and volume fillingfactor f, in which the `porosity length' h=l/f increases with local radius as h= h' r, we find that a substantial reduction in wind absorption requires aquite large porosity scale factor h' > 1, implying large porosity lengths h >r. The associated wind structure must thus have either a relatively large scalel~ r, or a small volume filling factor f ~ l/r << 1, or some combination ofthese. The relatively small-scale, moderate compressions generated by intrinsicinstabilities in line-driving seem unlikely to give such large porositylengths, leaving again the prospect of instead having to invoke a substantial(ca. factor 5) downward revision in assumed mass-loss rates.Comment: 6 pages in apj-emulate; 3 figures; submitted to Ap