Models of the Mass-ejection Histories of Pre-planetary Nebulae. III. The Shaping of Lobes by Post-AGB Winds

Abstract

Abstract We develop a physical framework for interpreting high-resolution images and kinematics of pre-planetary nebulae (“prePNe”). We use hydrodynamical models to infer the historical properties of fast collimated nuclear flows (“jet”) that successfully form hollow, candle-shaped lobes over ≈103 yr, including the density, momenta, and geometry of the jet and its environment. Next we vary the most influential parameters of this “baseline” model to investigate how changes in the flow parameters affect the model outcomes after 900 yr. Several generic conclusions emerge, such as the injected flows that create the hollow lobes must be light, “tapered,” and injected considerably faster than the lobe expansion speed. Multipolar and starfish prePNe probably evolve from wide-angle flows in which thin-shell instabilities corrugate their leading edges. We show how the common linear correlation of Doppler shift and position along the lobe is a robust outcome of the interaction of tapered diverging streamlines with the lobes’ curved walls. Finally, we probe how modest toroidal magnetic fields added to the fast flow affect the outcome of the baseline model. We conclude that the light, field-free, tapered baseline flow model is not only a successful and universal paradigm for unraveling the histories of lobe formation in prePNe, but also provides a comprehensive, inclusive framework for understanding the details of the shapes, sizes, and internal kinematics of their edges.