On the non-spherical structure of the nova nebulosity
The observed nonspherical structure of the material ejected during a nova outburst is explained by considering a component of the cataclysmic binaries ignored in previous theories - the ring of material which lies in the equatorial plane and surrounds the white dwarf. When the expanding shell collides with the ring, a shock is formed which heats the material in the ring and in the portion of the shell striking the ring. There is enough energy and density in the expanding shell for it to sweep the ring along, but with a final velocity somewhat lower than the shell moving perpendicular to the ring. The result is a prolate ellipsoid with the long axis perpendicular to the plane of the binary orbit. As the nebulosity evolves, the density remains greater in the equatorial plane than at the polar axis. This hypothesis is supported by calculations of the forbidden emission line ratios which also imply a higher equatorial density.