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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- ISM: molecules
- Infrared: ISM: lines and bands
- Planetary nebulae: general
- Planetary nebulae: individual (NGC 6720, NGC 7027)