FUSE Observations of Variability in Hot-Star Winds

New constraints on the large-scale wind structures responsible for discrete absorption components are obtained from far-ultraviolet time-series observations of O supergiants in the Large Magellanic Cloud. 1. What Causes DACs? Discrete absorption components (DACs) are optical depth enhancements in the absorption troughs of P Cygni profiles, which (a) occur very commonly in wind profiles of OB-type stars; (b) accelerate slowly compared with the expected wind flow; and (c) often recur on time scales related to stellar rotation. Since the radial optical depth at any position v ≡ v(r) in an expanding wind is given by the Sobolev approximation