H2 morphology of young planetary nebulae

The distributions of H2 1-0 S(1) emission in the young planetary nebulae BD + 30 deg 3639 and NGC 7027 show striking similarities: both have limb-brightened arcs of H2 emission with radii that are about twice those of their H II regions. The extended H2 emission in both nebulae is attributed to a photodissociation region, implying that the neutral envelopes of these young planetaries extend well beyond the edge of the H II region in contrast to older nebulae, where the ionized and molecular gas are more nearly coextensive. The contrast between young and old planetaries can only be explained if the molecular envelope is inhomogeneous. We endorse a scenario for the evolution of a planetary nebula in which a photodissociation front propagates through the clumpy molecular envelope, leaving the ionized core embedded in an envelope of partially ionized atomic gas and dense molecular knots.