Title: Herschel images of NGC 6720: H-2 formation on dust grains
Authors: van Hoof, P. A. M ×
Van de Steene, G. C
Barlow, M. J
Exter, Katrina
Sibthorpe, B
Ueta, T
Peris, V
Groenewegen, M. A. T
Blommaert, Joris
Cohen, M
De Meester, W
Ferland, G. J
Gear, W. K
Gomez, H. L
Hargrave, P. C
Huygen, Eric
Ivison, R. J
Jean, Christophe
Leeks, S. J
Lim, T. L
Olofsson, G
Polehampton, E. T
Regibo, Sara
Royer, Pierre
Swinyard, B. M
Vandenbussche, Bart
Van Winckel, Hans
Waelkens, Christoffel
Walker, H. J
Wesson, R #
Issue Date: Jul-2010
Publisher: Edp sciences s a
Series Title: Astronomy & astrophysics vol:518 pages:1-7
Article number: L137
Abstract: 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.
ISSN: 0004-6361
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Institute of Astronomy
× corresponding author
# (joint) last author

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