THE ENVELOPES OF BE STARS DRIVEN BY OPTICALLY THIN LINES

We investigate the dynamics of the envelopes of Be stars in the equato rial plane due to a force arising from a large number of optically thi n lines. This force is a possible candidate for driving a slowly expan ding wind because of its dependence on r. The force can be expressed a s F(w) = [GM(1 - GAMMA(e))/r2] W(r), where W(r) is a function of the t otal 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 so lution that increases from a subsonic speed at the surface of the star to a supersonic speed at larger distances. We therefore assume W(r) t o have the form eta(r/R)epsilon, where eta and epsilon are constant. T he constraint on the location of the sonic point provides some restric tion on the permitted values of epsilon and eta. We calculate the velo city distribution for different values of epsilon and eta. For the ran ge 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 wi th 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 = 0.49 5 and surface number density n = 1.8 x 10(14) cm-3 produce a grossly s ymmetric Halpha line profile with an equivalent width = 9.3 angstrom. This demonstrates that the weak line-driven wind model is capable of r eproducing some of the Halpha emission profiles seen in Be stars. Last ly we show that an axisymmetric stellar wind with an equatorial densit y enhancement can be produced by combining together an equatorial solu tion in which weak lines dominate the dynamics with a polar solution f or which strong lines dominate.