THE ENVELOPES OF BE STARS DRIVEN BY OPTICALLY THIN LINES

We investigate the dynamics of the envelopes of Be stars in the equatorial plane due to a force arising from a large number of optically thin lines. This force is a possible candidate for driving a slowly expanding wind because of its dependence on r. The force can be expressed as F(w) = [GM(1 - GAMMA(e))/r2] W(r), where W(r) is a function of the total number of weak lines and the average opacity of weak lines. If W(r) is a constant independent of r, one will not be able to obtain a solution that increases from a subsonic speed at the surface of the star to a supersonic speed at larger distances. We therefore assume W(r) to have the form eta(r/R)epsilon, where eta and epsilon are constant. The constraint on the location of the sonic point provides some restriction on the permitted values of epsilon and eta. We calculate the velocity distribution for different values of epsilon and eta. For the range of epsilon and eta we consider, the velocity distribution increases much more slowly than does the velocity for a strong line-driven wind, and the terminal velocity lies between 60 and 300 km s-1 compared with over 1000 km s-1 for a strong line-driven wind. Calculations for a wind driven by optically thin lines with epsilon = 0.01 and eta = 0.495 and surface number density n = 1.8 x 10(14) cm-3 produce a grossly symmetric Halpha line profile with an equivalent width = 9.3 angstrom. This demonstrates that the weak line-driven wind model is capable of reproducing some of the Halpha emission profiles seen in Be stars. Lastly we show that an axisymmetric stellar wind with an equatorial density enhancement can be produced by combining together an equatorial solution in which weak lines dominate the dynamics with a polar solution for which strong lines dominate.