The shaping of planetary nebulae and the SN 1987A nebula

The interacting winds model, in which the fast wind from a central hot star sweeps up a shell in a surrounding slow, dense wind, has been extensively applied to planetary nebulae and to the nebula around SN 1987A. If the wind properties are approximately constant over a period of time, the flow becomes self-similar. The additional assumption of a constant interior pressure allows the use of the thin shell approximation for the case of an axisymmetric slow wind density distribution. Calculations of this type show the importance of the velocity of the slow wind tb the nebular shape. The thin shell approximation can also be used in the case of an axisymmetric magnetic field in the shocked fast wind; the aspherical shape is then determined by the magnetic field of the central, rotating star. Photoionization by the central star can cause the swept-up shell to become geometrically thick. There is evidence that this occurred in the SN 1987A nebula and that the shock wave interaction with the thick shell produces the observed X-ray and radio emission from the nebula. The combination of photoionization and substantial density asymmetry can result in a flow of the ionized gas away from the equator. The flow can produce a quadrupolar symmetry in the nebular shape.