We investigate the degree to which moderate stellar rotation rates can influence the two-dimensional density structure in the winds of four classes of stars: Wolf-Rayet, B[e], asymptotic giant branch (AGB), and novae. These classes are distributed across the H-R diagram and have a wide range of escape speeds and wind acceleration. Furthermore, all have members which possess asymmetric circumstellar nebulae. It has been suggested that these asymmetries could result from stellar winds which have moderate equatorial density enhancements. Large enhancements may arise as the result of stellar rotation as demonstrated by the wind-compressed disk (WCD) model of Bjorkman & Cassinelli. Instead of a dense disk, here we consider a milder distortion called a wind-compressed zone (WCZ), A WCZ is said to occur if a star rotates more slowly than the disk formation threshold and if the density at the equator is more than about 3 times that at the pole. We assume that the stellar winds obey a standard β-velocity law and consider the effects of varying two of the velocity law parameters: the terminal speed, v_∞, and the exponent, β. For a given rotation rate, the wind compression is enhanced as either v_∞ is decreased or β is increased, because both correspond to a smaller acceleration of the wind. A general result from our model simulations is that the asymptotic density and flow structure are predominantly governed by the ratio ω/ω_D, where ω is the stellar rotation rate normalized to the critical speed and ω_D is the threshold value needed for disk formation. For the Wolf-Rayet and B[e] models which have moderate wind terminal speeds and shallow velocity laws (β= 3), a WCZ can form even at rotation rates of order 10% and 20% critical, respectively. For the AGB model with a low terminal speed and a β = 3 velocity law, a WCZ can form at 15% critical. Finally, we consider novae, which have time-variable wind properties. In particular, the location of the sonic point is time dependent, so we compute models with a range of sonic point radii. In favorable cases, a WCZ can form for white dwarf rotation rates of less than 20% critical; however, further work will be required to properly treat the extended subsonic region of nova winds.
展开▼
