Author:

Keywords:

circumstellar matter, cloudy, dust, extinction, emission, helix-nebula, infrared: ism, ism: molecules, knots, molecular-hydrogen, planetary nebulae: individual: ngc 6720, planetary-nebulae, ring nebula, size distributions, spectra, star-forming regions, Science & Technology, Physical Sciences, Astronomy & Astrophysics, planetary nebulae: individual: NGC 6720, infrared: ISM, ISM: molecules, STAR-FORMING REGIONS, MOLECULAR-HYDROGEN, PLANETARY-NEBULAE, HELIX-NEBULA, RING NEBULA, SIZE DISTRIBUTIONS, SPECTRA, KNOTS, EMISSION, CLOUDY, astro-ph.SR, 0201 Astronomical and Space Sciences, 5101 Astronomical sciences, 5107 Particle and high energy physics, 5109 Space sciences

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.