The correlation between expansion velocity and morphology in planetary nebulae

Abstract

It is usually accepted that the differing morphological classes of planetary nebulae (PNe) arise from progenitors of differing mass. The primary evidence for this derives from the differing galactic distributions of the sources. This, if true, would be expected to result in other differences as well, including variations in the kinematics of the nebular envelopes. We point out here that there is now sufficient evidence to determine that this is the case. We find that BRET-type sources (i.e. nebulae possessing “bipolar rotating episodic jets") have the lowest velocities of expansion VEXP, followed (in order of increasing velocity) by bipolar (BPNe), elliptical and circular nebulae. In addition to this, we find that the distributions of circular, elliptical and bipolar sources are quite distinct, with BPNe being biased towards lower velocities, and circular sources distributed more uniformly. It appears therefore that bipolar outflows contain, within the same shells, evidence for both the highest and lowest velocities of expansion. Whilst the outer wings of these nebulae are expanding at ~175 km s-1, the brighter parts of the shells (probably corresponding to equatorial toroids) have velocities of only ~18 km s-1.