Herschel images of NGC 6720: H2 formation on dust grains

TL;DR

This study uses Herschel images and photoionization modeling to provide observational evidence that molecular hydrogen forms on dust grains within the planetary nebula NGC 6720, especially in high-density knots formed after gas recombination.

Contribution

It demonstrates the correlation between dust and H2 emission in NGC 6720 and proposes a new formation scenario involving hydrodynamical instabilities and dense knots.

Findings

H2 forms on dust grains in the nebula

H2 resides in high-density knots formed after recombination

H2 formation may still be ongoing today

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.