EQUATORIAL WIND COMPRESSION EFFECTS ACROSS THE H-R DIAGRAM

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 be en suggested that these asymmetries could result from stellar winds wh ich have moderate equatorial density enhancements. Large enhancements may arise as the result of stellar rotation as demonstrated by the win d-compressed disk (WCD) model of Bjorkman & Cassinelli. Instead of a d ense disk, here we consider a milder distortion called a wind-compress ed zone (WCZ). A WCZ is said to occur if a star rotates more slowly th an the disk formation threshold and if the density at the equator is m ore than about 3 times that at the pole. We assume that the stellar wi nds obey a standard beta-velocity law and consider the effects of vary ing two of the velocity law parameters: the terminal speed, upsilon(in finity), and the exponent, beta. For a given rotation rate, the wind c ompression is enhanced as either upsilon(infinity) is decreased or bet a is increased, because both correspond to a smaller acceleration of t he wind. A general result from our model simulations is that the asymp totic density and flow structure are predominantly governed by the rat io omega/omega(D), where omega is the stellar rotation rate normalized to the critical speed and omega(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 (beta=3), a WCZ can fo rm even at rotation rates of order 10% and 20% critical, respectively. For the AGE model with a low terminal speed and a beta=3 velocity law , a WCZ can form at 15% critical. Finally, we consider novae, which ha ve time-variable wind properties. In particular, the location of the s onic point is time dependent, so we compute models with a range of son ic point radii. In favorable cases, a WCZ can form for white dwarf rot ation rates of less than 20% critical; however, further work will be r equired to properly treat the extended subsonic region of nova winds.