2012ApJ...755..136W

We present one-dimensional hydrodynamical models that suggest a new mechanism for generating spiral structures around evolved stars. In these models, the pulsation of the star initiates the mass outflow and generates shock waves from a static atmosphere. An orbiting companion perturbs the outflow, creating a long-period series of shocks with different speeds and gas density that form spiral arms around the star. The spacing and number of spirals generated are predictable from pulsation and orbital periods. Companions in close orbits create up to four spiral arms, while companions farther out predominantly form single spiral arms. The interaction also produces a variety of density and temperature structures close to the star in the equatorial plane. This study extends the set of mechanisms for explaining the morphologies of protoplanetary nebulae.