Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003pasp..115..170s&link_type=abstract
The Publications of the Astronomical Society of the Pacific, Volume 115, Issue 804, pp. 170-177.
Astronomy and Astrophysics
Astronomy
24
Stars: Circumstellar Matter, Ism: Planetary Nebulae: Individual: Ngc Number: Ngc 6720, Planetary Nebulae: Individual (M57), Stars: Agb And Post-Agb, Stars: Evolution, Stars: Mass Loss
Scientific paper
We present a 0.65" resolution H2 1-0 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 H2 emission, suggesting that these clumps are similar to the cometary knots seen in the Helix Nebula. As with the Helix, the clumpy H2 emission 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 H2 average surface brightnesses for the Ring Nebula [(1.5+/-0.5)×10-4 ergs cm-2 s-1 sr-1] with time-dependent models of the expected H2 emission from planetary nebulae (PNe) shows that it is consistent with H2 excitation in photodissociation regions (PDRs), confirming previous suggestions. Comparison of the Ring Nebula H2 emission with a younger PN, NGC 2346, and an older PN, the Helix Nebula, suggests an evolution in H2 surface 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.
Jacoby George H.
Knezek Patricia M.
Meixner Margaret
Speck Angela Karen
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