000002291 001__ 2291
000002291 005__ 20160706142048.0
000002291 0247_ $$2DOI$$a10.1051/0004-6361/201014590
000002291 037__ $$aASTROimport-448
000002291 100__ $$avan Hoof, P. A. M.
000002291 245__ $$aHerschel images of NGC 6720: H2 formation on dust grains
000002291 260__ $$c2010
000002291 520__ $$aHerschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H2 emission, which appears to be observational evidence that H2 forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of the dust and investigate possible formation scenarios for the H2. We conclude that the most plausible scenario is that the H2 resides in high density knots which were formed after the recombination of the gas started when the central star entered the cooling track. Hydrodynamical instabilities due to the unusually low temperature of the recombining gas are proposed as a mechanism for forming the knots. H2 formation in the knots is expected to be substantial after the central star underwent a strong drop in luminosity about one to two thousand years ago, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Figures 2-6 and Appendices are only available in electronic form at http://www.aanda.org
000002291 700__ $$avan de Steene, G. C.
000002291 700__ $$aBarlow, M. J.
000002291 700__ $$aExter, K. M.
000002291 700__ $$aSibthorpe, B.
000002291 700__ $$aUeta, T.
000002291 700__ $$aPeris, V.
000002291 700__ $$aGroenewegen, M. A. T.
000002291 700__ $$aBlommaert, J. A. D. L.
000002291 700__ $$aCohen, M.
000002291 700__ $$aDe Meester, W.
000002291 700__ $$aFerland, G. J.
000002291 700__ $$aGear, W. K.
000002291 700__ $$aGomez, H. L.
000002291 700__ $$aHargrave, P. C.
000002291 700__ $$aHuygen, E.
000002291 700__ $$aIvison, R. J.
000002291 700__ $$aJean, C.
000002291 700__ $$aLeeks, S. J.
000002291 700__ $$aLim, T. L.
000002291 700__ $$aOlofsson, G.
000002291 700__ $$aPolehampton, E. T.
000002291 700__ $$aRegibo, S.
000002291 700__ $$aRoyer, P.
000002291 700__ $$aSwinyard, B. M.
000002291 700__ $$aVandenbussche, B.
000002291 700__ $$avan Winckel, H.
000002291 700__ $$aWaelkens, C.
000002291 700__ $$aWalker, H. J.
000002291 700__ $$aWesson, R.
000002291 773__ $$cL137$$pAstronomy and Astrophysics$$v518$$y2010
000002291 85642 $$ahttp://esoads.eso.org/abs/2010A%26A...518L.137V
000002291 905__ $$apublished in
000002291 980__ $$aREFERD