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Herschel images of NGC 6720: H2 formation on dust grains
van Hoof, P.A.M.; Van de Steene, G.C.; Barlow, M.J.; Exter, K.M.; Sibthorpe, B.; Ueta, T.; Peris, V.; Groenewegen, M.A.T.; Blommaert, J.A.D.L.; Cohen, M.; De Meester, W.; Ferland, G.J.; Gear, W.K.; Gomez, H.L.; Hargrave, P.C.; Huygen, E.; Ivison, R.J.; Jean, C.; Leeks, S.J.; Lim, T.L.; Olofsson, G.; Polehampton, E.T.; Regibo, S.; Royer, P.; Swinyard, B.M.; Vandenbussche, B.; Van Winckel, H.; Waelkens, C.; Walker, H.J.; Wesson, R. (2010). Herschel images of NGC 6720: H2 formation on dust grains. Astron. Astrophys. 518: L137. https://dx.doi.org/10.1051/0004-6361/201014590
In: Astronomy & Astrophysics (Les Ulis). EDP Sciences: Les Ulis. ISSN 0004-6361; e-ISSN 1432-0746, more
Peer reviewed article  
Available in  Authors 
Author keywords
    planetary nebulae: individual: NGC 6720 / circumstellar matter / dust, extinction / infrared: ISM / ISM: molecules
Authors  Top 
  • van Hoof, P.A.M.
  • Van de Steene, G.C.
  • Barlow, M.J.
  • Exter, K.M., more
  • Sibthorpe, B.
  • Ueta, T.
  • Peris, V.
  • Groenewegen, M.A.T.
  • Blommaert, J.A.D.L.
  • Cohen, M.
  • De Meester, W.
  • Ferland, G.J.
  • Gear, W.K.
  • Gomez, H.L.
  • Hargrave, P.C.
  • Huygen, E.
  • Ivison, R.J.
  • Jean, C.
  • Leeks, S.J.
  • Lim, T.L.
  • Olofsson, G.
  • Polehampton, E.T.
  • Regibo, S.
  • Royer, P.
  • Swinyard, B.M.
  • Vandenbussche, B.
  • Van Winckel, H.
  • Waelkens, C.
  • Walker, H.J.
  • Wesson, R.
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 H2 emission, which appears to be observational evidence that H2 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 H2. We conclude that the most plausible scenario is that the H2 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. H2 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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