Three Dimensional Magneto-Hydrodynamical Modeling of Planetary Nebuale ~ II: The Formation of Bipolar and Elliptical Nebulae with Point-Symmetric Structures and Collimated Outflows

Abstract

This work presents three dimensional, magneto-hydrodynamical simulations of the formation and early evolution of a subgroup of planetary nebulae that exhibit a variety of point-symmetric structures. For bipolar nebulae, the formation of point-symmetric nebular shapes along the inner borders of their opposing lobes and/or collimated outflows or jets internal or external to their cavities, is reproduced by considering a steady misalignment of the magnetic collimation axis with respect to the symmetry axis of the bipolar wind outflow, defined perpendicular to the equatorial density enhancement. Elliptical planetary nebulae with ansae displaced from the symmetry axis in point-symmetric fashion are reproduced through the same process by reducing the equatorial density enhancement. This mechanism represents an alternative explanation to some cases where morphological appearances give the impression of the action of a symmetric, rotating or precessing jet from the central source. The computational survey reveals that jet formation is detected only for dense enough winds with mass loss rates $\gtrsim 10^{-7}$ \Moy. 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 velocity, since magnetized bubbles behave adiabatically for low wind velocities ($\simeq$ $100 \kms$), which in the absence of a magnetic field would behave as momentum driven.