The evolving morphology of the bipolar nebula M2-9

The planetary nebula M2-9 has drastically changed its shape since its discovery by Minkwoski. Although the outline of the nebula seems to be stationary, most of the knots and bright features (N1, N2, S1, and S2) have moved laterally from the west to the east edges of both lobes. These features and their changes have reflection, not point, symmetry. We have compiled high-quality CCD images in H alpha and [O III] obtained every 2-5 years since 1985 to monitor position and morphological changes in the individual knots. Our results show that the recent structural changes are more complex than suggested previously. The pattern of changes resemble a rotating corkscrew-like pattern, as if a precessing ionization/excitation beam inscribes the knots and filaments on the pencil-shaped lobe edges. The beam flow speed is no higher than 0.01c, so the beam is not a light beam. Its rotation period is about 120 yr. The corkscrew pattern and its apparent speed render many simple models implausible. The model of beads sliding along a helical wire is easily ruled out by observations. A model that may work is a combination of a beam and a spray of energetic particles, both invisible and moving radially at about 1000 km s(-1), which shock-heats and ionizes the walls of the lobes as the beams precess.