ON THE SHAPING OF CIRCUMSTELLAR ENVELOPES

In this paper, we examine the evolutionary process of expansion of a m agnetized ring of cool matter ejected from a red giant. The subsequent formation of a circumstellar shell or preplanetary nebula is also ana lyzed. The velocity, density, and magnetic field are calculated along a time sequence, on a large two-dimensional grid of points close to an d far away from the star. Owing to long-range forces such as thermal p ressure gradients and magnetic stresses, the velocity field becomes a linear function of the distance. The latitudinal dependence of this fu nction is poor, while conversely the density distribution is clearly a spheric. The magnetic field intensity at a distance approximately 10(1 5)-10(16) cm is in good agreement with the observational data. The iso photes are plotted and compared to observational contours obtained fro m the literature. The filamentary structure appearance and its stabili ty within the envelope are also discussed.