A Speculation into the Origin of Neutral Globules In Planetary Nebulae: Could the Helix's Comets Really Be Comets?

Abstract

A novel explanation for the origin of the cometary globules within NGC 7293 (the "Helix" planetary nebula) is examined; that these globules originate as massive cometary bodies at large astrocentric radii. The mass of such hypothetical cometary bodies would have to be several orders of magnitude larger than any such bodies observed in our solar system in order to supply the observed mass of neutral gas. It is however shown that comets at "outer Oort cloud" like distances are likely to survive past the red giant and asymptotic giant branch evolutionary phases of the central star, allowing them to survive until the formation of the planetary nebula. Some observational tests of this hypothesis are proposed.