Classification of Planetary Nebulae by their Departure from Axisymmetry

Abstract

We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e., between the two sides perpendicular to the symmetry axis of the nebula. We consider 6 types of departure from axisymmetry: (1) PNe whose central star is not at the center of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe whose symmetry axis is bent, e.g., the two lobes in bipolar PNe are bent toward the same side; (5) PNe whose main departure from axisymmetry is in the outer regions, e.g., an outer arc; (6) PNe which show no departure from axisymmetry, i.e., any departure, if it exists, is on scales smaller than the scale of blobs, filaments, and other irregularities in the nebula. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly due to the influence by a stellar binary companion. We find that about 50 percents of all PNe possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely 25-30 percents. We argue that this discrepancy comes from many PNe whose departure from axisymmetry, mainly unequal size, shape, or intensity, results from the presence of long-lived and large, hot or cool, spots on the surface of their AGB progenitors. Such spots locally enhance mass loss rate, leading to a deparure from axisymmetry, mainly near the equator, in the descendant PN.