The bipolar symmetry of ring-like planetary nebulae: Molecular hydrogen emission from halos

To investigate the relationship between H₂ morphology, optical halos, and the structure of planetary nebulae (PNs), we used the Cryogenic Optical Bench at Kitt Peak National Observatory to obtain deep images of representative objects in the 2.122 v = 1 → 0, S(1) line of H₂. We find halo H₂ emission in NGC 6720 (the Ring Nebula), in Ring-like NGC 6772 and 6781, and in bipolar NGC 2346. In general, H₂ halos precisely outline optical halos, but H₂ core-to-halo surface brightness ratios are much smaller than those of their optical counterparts. Shock excitation appears to be a more likely mechanism than UV excitation for producing H₂ halos in these evolved PNs. Geometric considerations then 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 H₂, He I, and Brγ that vividly demonstrate the spatial separation of molecular and ionized emission previously established for this PN. The detached and morphologically dissimilar H₂ and ionized emitting regions support the UV fluorescence model for this young PN.