Herschelimages of NGC 6720: H₂formation on dust grains

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.

Herschelimaging of the dust in the Helix nebula (NGC 7293)

“…Molecular hydrogen formation in the knots is expected to be substantial after the central star entered the cooling track and underwent a strong drop in luminosity, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots and assuming the knots formed quickly. Especially in the latter case we expect a tight correlation between the dust emission and the H 2 emission (van Hoof et al 2010) as is observed in the Helix nebula.…”

“…Goldsmith (2001) found that for gas densities higher than 10 4.5 cm −3 the dust and gas temperatures will be closely coupled, also for temperatures as found in the Helix. It is another indication that, as is the case in the Ring Nebula (van Hoof et al 2010) andNGC 650 (van Hoof et al 2013), also in NGC 7293 H 2 and dust co-exist in these knots. This close correlation could be an indication of the formation of H 2 on dust grains.…”

“…These knots may be density enhancements formed during the AGB phase in the shell and survived the ionization of the nebula (Matsuura et al 2009;Aleman et al 2011), or they may have formed because of instabilities and fragmentation at the ionization front when the nebula got ionized (García-Segura et al 2006;Huggins & Frank 2006). As knots are observed in older PNe having central stars well evolved on the cooling track, it has been suggested that they formed because of hydrodynamical instabilities when the nebular gas is going through recombination (O'Dell et al 2007;van Hoof et al 2010). The formation of H 2 most effectively takes place on the surface of dust grains, also at grain temperatures as found here for the Helix nebula (Cazaux & Tielens 2002.…”

“…The high density in the knots makes it possible to shield the H 2 from the stellar radiation field. The high densities and low dust temperatures observed allow H 2 molecules to be reformed on dust grains on a reasonable timescale (van Hoof et al 2010). Molecular hydrogen formation in the knots is expected to be substantial after the central star entered the cooling track and underwent a strong drop in luminosity, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots and assuming the knots formed quickly.…”

Near-infrared photometry of Galactic planetary nebulae with the VVV Survey

“…To further test the emission mechanisms contributing to the VVV bands, we made use of the published photo-ionisation model of van Hoof et al (2010) for NGC 6720, the Ring nebula. This is the archetypal planetary nebula; the nebula is bipolar, seen almost pole-on, optically thick to ionizing radiation except towards the poles.…”