Three-dimensional magnetohydrodynamic modeling of planetary nebulae. II. The formation of bipolar and elliptical nebulae with point-symmetric structures and collimated outflows

This work presents three-dimensional, magnetohydrodynamic simulations of th e formation and early evolution of a subgroup of planetary nebulae that exh ibit a variety of point-symmetric structures. For bipolar nebulae, the form ation of point-symmetric nebular shapes along the inner borders of their op posing lobes, and/or collimated outflows or jets internal or external to th eir cavities, is reproduced by considering a steady misalignment of the mag netic collimation axis with respect to the symmetry axis of the bipolar win d outflow, defined perpendicular to the equatorial density enhancement. Ell iptical planetary nebulae with ansae displaced from the symmetry axis in po int-symmetric fashion are reproduced through the same process by reducing t he equatorial density enhancement. This mechanism represents an alternative explanation to some cases where morphological appearances give the impress ion of the action of a symmetric, rotating or precessing jet from the centr al source. The computational survey reveals that jet formation is detected only for dense enough winds with mass-loss rates greater than or similar to 10(-7) M. yr(-1) For lower mass-loss rates the jets tend to vanish leaving behind only ansae-like structures at the tips of the lobes, as observed in some cases. The results are rather independent of the wind terminal veloci ty, since magnetized bubbles behave adiabatically for low wind velocities ( similar or equal to 100 km s(-1)), which in the absence of a magnetic field would behave as momentum driven.