Letter to the Editor

Herschel images of NGC 6720: H₂ formation on dust grains ^*,**

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium e-mail: p.vanhoof@oma.be
² Dept of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Dept. of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA
⁶ Astronomical Observatory, Valencia University, Edifici Instituts d'Investigació, Parc Científic, C/ Catedrático Agustín Escardino 7, 46980 Paterna (Valencia), Spain

⁷ Radio Astronomy Laboratory, University of California at Berkeley, CA 94720, USA
⁸ University of Kentucky, Dept. of Physics and Astronomy, 177 CP Building, Lexington, KY 40506–0055, USA
⁹ School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, Wales CF24 3YB, UK
¹⁰ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
¹¹ Dept. of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden
¹² Department of Physics, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada

Received: 30 March 2010
Accepted: 5 April 2010

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.