Can planetary nebulae rotate ?

The inclination observed in the spectral lines of a number of planetary nebulae when the spectrograph slit is placed along the major axis may be due to the rotation of the nebulae about their minor axes, as suggested by Campbell and Moore (1916, 1918). It is assumed that the rotation of the ventral star serves as the source of the original rotation of a protoplanetary nebula. The mechanism strengthening the original rotation in the process of expansion of the shell is the tangential pressure of L(alpha) radiation due to the anisotropic properties of the medium and radiation field. The dynamic effect of these properties appears to be greatest in the epoch when the optical depth of the nebula in the L(alpha) continuum comes to be of the order of unity in the course of its expansion.