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 nu = 1 goes to 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 ultraviolet (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 Br-gamma 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 flourescence model for this young PN.