SIMBAD references

We present a 0".65 resolution H₂1-0 S(1) 2.122 µm image of the Ring Nebula (NGC 6720), which was taken with the Near Infrared Imager at the WIYN 3.5 m telescope on Kitt Peak. The high resolution of the H₂ observation is sufficient to reveal the finer structure of the molecular material in this nebula. The morphology of the molecular emission is compared to that of the ionized emission from the Ring Nebula as seen by the Hubble Space Telescope (HST; He II, [O III], and [N II]), and it is clear that the dark clumps seen by HST match the locations of clumpy H₂emission, suggesting that these clumps are similar to the cometary knots seen in the Helix Nebula. As with the Helix, the clumpy H₂emission from the main ring of the Ring Nebula is contained within the optically bright ionized nebula, implying that the molecular gas is shielded inside dense condensations. Comparison of the observed H₂average surface brightnesses for the Ring Nebula [(1.5±0.5)x10^–4 ergs.cm^–2.s^–1.sr^–1] with time-dependent models of the expected H₂emission from planetary nebulae (PNe) shows that it is consistent with H₂excitation in photodissociation regions (PDRs), confirming previous suggestions. Comparison of the Ring Nebula H₂emission with a younger PN, NGC 2346, and an older PN, the Helix Nebula, suggests an evolution in H₂surface brightness consistent with the time-dependent PDR models. Moreover, the knots of molecular gas appear to become more isolated as the PN evolves, consistent with optical studies of knots in PNe.