Molecular Hydrogen Emission from the Cometary Globules in the Helix Nebula (NGC 7293)

Extract

The discovery of many dense, dusty condensations in the Helix Nebula, NGC 7293, by Meaburn et al. (1992) was the first direct evidence of the real inhomogeneity of the medium of a planetary nebula. On account of the small distance of the Helix nebula from the Sun (~200pc), the knots (cometary globules) can be resolved from the ground and studied in detail from HST imaging (O'Dell & Handron, 1996). The condensations typically have a projected diameter of <2″ and hence sizes of <6 x 10¹⁵ cm. The condensations consist of a dusty core, visible as absorption against the background high ionization central region of the nebula for the foreground globules, and with a bow-shaped ionization front, strong in low ionization emission. The emission is displaced in the direction towards the central star, often with an outwardly-directed radial tail.