H-2 MORPHOLOGY OF YOUNG PLANETARY-NEBULAE

The distributions of H-2 1-0 S(1) emission in the young planetary nebu lae BD +30-degrees 3639 and NGC 7027 show striking similarities: both have limb-brightened arcs of H-2 emission with radii that are about tw ice those of their H II regions. The extended H-2 emission in both neb ulae is attributed to a photodissociation region. This implies that th e neutral envelopes of these young planetaries extend well beyond the edge of the H II region, in contrast to older nebulae where the ionize d and molecular gas are more nearly coextensive. The contrast between young and old planetaries can only be explained if the molecular envel ope is inhomogeneous. We endorse a scenario for the evolution of a pla netary nebula in which a photodissociation front propagates through th e clumpy molecular envelope, leaving the ionized core embedded in an e nvelope of partially ionized atomic gas and dense molecular knots. In an evolved planetary, the H II region has expanded to engulf some of t he dense molecular knots, which can be identified with bright [O I] an d H-2 1-0 S(1) condensations, while the remnant of the photodissociate d envelope may be detected as a faint optical halo.