A morphological study of planetary nebulae

Abstract

We have produced simulated images of 110 planetary nebulae using the ellipsoidal shell model. This process has allowed us to remove the projection effects from the morphological classification of planetary nebulae and has provided quantitative measures of the intrinsic asymmetries of the nebulae. It is shown that the morphology of most planetary nebulae can be reproduced with pole-to-equator density ratios of 0.1-1. Many planetary nebulae also show a modest departure from axial symmetry. Contrary to previous findings by Khromov & Kohoutek, the sky orientation of planetary nebulae in this sample is consistent with a purely random distribution. Extremely bipolar nebulae (e.g., those of butterfly shape) point to a steep density profile in the AGB envelope and are more likely to be type I (high helium and/or nitrogen abundance) nebulae. We found evidence that these nebulae are likely to have more massive progenitors and are at a more advanced stage of dynamical evolution.