THE BIPOLAR SYMMETRY OF RING-LIKE PLANETARY-NEBULAE - MOLECULAR-HYDROGEN EMISSION FROM HALOS

To investigate the relationship between H, morphology, optical halos, and the structure of planetary nebulae (PNs), we used the Cryogenic Op tical Bench at Kitt Peak National Observatory to obtain deep images of representative objects in the 2.122 v = 1 --> 0, S(1) line of H-2. We find halo H-2 emission in NGC 6720 (the Ring Nebula), in Ring-like NG C 6772 and 6781, and in bipolar NGC 2346. In general, H-2 halos precis ely outline optical halos, but H-2 core-to-halo surface brightness rat ios are much smaller than those of their optical counterparts. Shock e xcitation appears to be a more likely mechanism than UV excitation for producing H-2 halos in these evolved PNs. Geometric considerations th en suggest that these PNs are all bipolar in structure and viewed at a range of inclination angles from nearly perpendicular to nearly pole- on. We also present images of NGC 7027 in H-2, He I, and Br gamma that vividly demonstrate the spatial separation of molecular and ionized e mission previously established for this PN. The detached and morpholog ically dissimilar H-2 and ionized emitting regions support the UV fluo rescence model for this young PN.