H-2 MORPHOLOGY OF YOUNG PLANETARY-NEBULAE

The distributions of H-2 1-0 S(1) emission in the young planetary nebulae BD +30-degrees 3639 and NGC 7027 show striking similarities: both have limb-brightened arcs of H-2 emission with radii that are about twice those of their H II regions. The extended H-2 emission in both nebulae is attributed to a photodissociation region. This implies 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. In an evolved planetary, the H II region has expanded to engulf some of the dense molecular knots, which can be identified with bright [O I] and H-2 1-0 S(1) condensations, while the remnant of the photodissociated envelope may be detected as a faint optical halo.