The problem
Simulation of rotating density structures due to changing polarities in interplanetary magnetic field, first directly observed by the Ulysses spacecraft at our sun. Sudden velocity changes on the sun surface from coronal holes and the solar rotation introduce the spiral structure (see also Mullan 1984).
Smulation (density) of massive star CIRs (Cranmer & Owocki 1996, ApJ, 462, 469; Fig. 5).

Very massive stars spend most of their lifetimes in hot phases with strong winds. By far the strongest winds occur during the WR phase, making WR stars key objects to understand how stellar winds in general behave. Contrary to original assumptions, hot-star winds are not smooth. There are basically types of wind features that are known to occur:

  1. stochastic structures on a variety of small scales, often referred to as clumps, and

  2. a small number of large-scale spiral-like features that corotate with the underlying star, referred to as Corotating Interaction Regions (CIR) and manifesting themselves as the famous moving DACs (Discrete Absorption Components) seen in virtually all UV spectra of O-type stars (Kaper et al. 1999, A&A, 344, 231). Corotating Interaction Regions (CIRs) were first observed in the Sun (see Mullan 1984). They occur when a perturbation at the base of a wind propagates in it and is carried around by the rotation of the star, generating spiral-like regions of higher and lower density in the wind.

Recently, it has been found that some WR stars also have CIRs (St-Louis et al. 2009, ApJ, 698, 1951).

The clumpy structure probably reflects multi-scale turbulence, with mechanical eddies cascading from a few at large-scale to progressively more and more at smaller scales before the energy is dissipated (Moffat et al. 1994, Ap.Sp.Sc., 216, 55), as seen in many astrophysical situations (Henriksen 1991, ApJ, 377, 146). Turbulence always requires a driver. In stellar winds, this may be either radiative wind instabilities (Owocki et al. 1988, ApJ, 335, 914) or random convective motions reaching the stellar surface (Cantiello et al. 2009, A&A, 499, 279) and being transmitted to the wind at its base. Astronomers have been struggling for over two decades to understand wind clumping, with only modest success so far: We still do not know how to properly quantify these ubiquitous stochastic structures. However, not allowing for clumping will ultimately falsify the estimates of mass-loss rates, which are crucial for understanding stellar evolution of massive stars.

Unlike in ordinary stars, where we can see and measure the stellar surface, the presence of CIRs, which are thought to be pinned to the surface of the stars, in WR winds in WR winds is the only link we have to determine the rotation of the underlying star, which is normally not seen below the dense wind. Constraining the rotation of any star is important, but this is especially so for WR stars, the fastest rotating of which are thought to be the progenitors of the long Gamma Ray Bursts (GRB), which are associated with collapse supernovae (SNe) of type Ib (from WNstars?) and Ic (from WC/WO stars?). During the brief burst, the energy emitted makes the object outshine the whole rest of the Universe put together! Clearly one would like to understand these exciting objects in more detail. Even if only a very small fraction (~0.001) of WR stars rotate fast enough to be a serious candidate for a long GRB, it behooves us to gain detailed understanding of rotation in WR stars.