As wind parcels from opposite hemispheres collide over the equator, they form a disk of compressed gas. This WCD model provides a natural explanation for the strong disk emission lines seen from various classes of rapidly rotating hot stars. Color indicates gas density. These models ignored gravity darkening, which may inhibit WCDs. However, another aspect of rapid rotation complicates the situation.
Colors indicate the stars' surface radiation flux.
Their more recent simulations that include gravity darkening and nonradial driving forces now indicate that such additional effects can actually inhibit formation of a WCD and even cause the bright poles to drive a denser wind than the dimmer equator. But the general WCD paradigm is so compelling that it has since been invoked to understand circumstellar matter in a variety of other contexts. It's just a bit ironic that it may not apply to the radiatively driven stellar winds for which it was originally developed. More than a decade ago, Dermott Mullan, another Bartol researcher, suggested that similar CIRs in the wind outflows of hot stars might explain variations of certain absorption lines in their spectra. Recent hydrodynamical simulations by Cranmer and Owocki show that any region of enhanced brightness on a hot star would generate dense, low-speed wind streams.