Spherical planetary nebulae

By examining their mass loss history and their distribution in the galaxy, I argue that spherical planetary nebulae (PNe) form a special group among all planetary nebulae. The smooth surface brightness of most spherical PNe suggests that their progenitors did not go through a final intensive wind (FIW, also termed superwind) phase. While ~70% of the PNe of all other PNe groups are closer to the galactic center than the sun is, only ~30% of spherical PNe are; ~70% of them are farther away from the galactic center. These, plus the well-known high scale height above the galactic plane of spherical PNe, suggest that the progenitors of spherical PNe are low mass stars having low metallicity. Although many stars have these properties, only ~10% of all PNe are spherical. By comparing the galactic distribution of spherical PNe to the metallicity evolution in the galaxy, I find that the critical metallicity above which no spherical PNe are formed is ${\rm [Fe/H]} \sim -0.4$. I explain this as well as other properties of spherical PNe in the context of the companion model for shaping PNe, arguing that spherical PNe are formed from stars that have no close companion, stellar or substellar, orbiting them. I discuss the connection of the proposed scenario to the recent finding of extrasolar planets and to the presence of blue horizontal branch stars in globular clusters