The most direct evidence for the existence of asphericities in stellar winds is the intrinsic polarization observed in many hot stars.
Rotation can also cause equatorial density enhancements.
Thus, the survival of dust around these hot stars is a puzzling theoretical problem and several explanations have been proposed. The common feature of these models that permits dust formation is the existence of a high density region where the material is either shielded from the direct stellar radiation field or the high density increases the radiative cooling rate, enabling the local temperature to drop below the dust formation threshold. No matter whether this region arises due to compression of two colliding stellar winds, clumping in the wind of a single star, or an equatorially compressed zone, a thorough investigation of this problem requires non-spherical radiation transfer. O and B Stars Other systems that clearly require non-spherical radiation transfer are rapidly rotating O and B stars. Given that the rotation rates of Be stars are so high, it is quite likely that the mass loss is not spherically symmetric, so model line profiles really should be calculated using an axisymmetric model in order to produce more reliable mass loss rates.