Modeling ultraviolet wind line variability in massive hot stars

We model the detailed time-evolution of discrete absorption components (DACs) observed in P Cygni profiles of the Si IV l1400 resonance doublet lines of the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption that the DACs are caused by corotating interaction regions (CIRs) in the stellar wind. We perform 3D radiative transfer calculations with hydrodynamic models of the stellar wind that incorporate these large-scale density-and velocity-structures. We develop the 3D transfer code Wind3D to investigate the physical properties of CIRs with detailed fits to the DAC shape and morphology. We constrain the properties of large-scale wind structures with detailed fits to DACs observed in HD 64760. A model with two spots of unequal brightness and size on opposite sides of the equator-20% +/- 5% and 8% +/- 5% brighter than the stellar surface and with opening angles of 20 degrees +/- 5 degrees and 30 degrees +/- 5 degrees diameter, respectively-provides the best fit to the observed DACs. The recurrence time of the DACs compared to the estimated rotational period corresponds to spot velocities that are 5 times slower than the rotational velocity. The massloss rate of the structured-wind model for HD 64760 does not exceed the rate of the spherically symmetric smooth-wind model by more than 1%. The fact that DACs are observed in a large number of hot stars constrains the clumping that can be present in their winds, as substantial amounts of clumping would tend to destroy the CIRs.