Journal
ASTRONOMY & ASTROPHYSICS
Volume 518, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201014590
Keywords
planetary nebulae: individual: NGC 6720; circumstellar matter; dust, extinction; infrared: ISM; ISM: molecules
Categories
Funding
- Belgian Federal Science Policy Office of ESA
- BMVIT (Austria)
- ESA-PRODEX (Belgium)
- CEA (France)
- CNES (France)
- DLR (Germany)
- ASI (Italy)
- INAF (Italy)
- CICYT (Spain)
- MCYT (Spain)
- CSA (Canada)
- NAOC (China)
- CNRS (France)
- MCINN (Spain)
- SNSB (Sweden)
- STFC (UK)
- NASA (USA)
- STFC [ST/G002827/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/G002827/1, PP/E001181/1] Funding Source: researchfish
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1109061] Funding Source: National Science Foundation
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Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H-2 emission, which appears to be observational evidence that H-2 forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of the dust and investigate possible formation scenarios for the H-2. We conclude that the most plausible scenario is that the H-2 resides in high density knots which were formed after the recombination of the gas started when the central star entered the cooling track. Hydrodynamical instabilities due to the unusually low temperature of the recombining gas are proposed as a mechanism for forming the knots. H-2 formation in the knots is expected to be substantial after the central star underwent a strong drop in luminosity about one to two thousand years ago, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots.
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