Modelling the warm H2 infrared emission of the Helix nebula cometary knots

Molecular hydrogen emission is commonly observed in planetary nebulae. Images taken in infrared H2 emission lines show that at least part of the molecular emission is produced inside the ionized region. In the best studied case, the Helix nebula, the H2 emission is produced inside cometary knots (CKs), comet-shaped structures believed to be clumps of dense neutral gas embedded within the ionized gas. Most of the H2 emission of the CKs seems to be produced in a thin layer between the ionized diffuse gas and the neutral material of the knot, in a mini-photodissociation region (mini-PDR). However, PDR models published so far cannot fully explain all the characteristics of the H2 emission of the CKs. In this work, we use the photoionization code Aangaba to study the H2 emission of the CKs, particularly that produced in the interface H+/H0 of the knot, where a significant fraction of the H2 1-0 S(1) emission seems to be produced. Our results show that the production of molecular hydrogen in such a region may explain several characteristics of the observed emission, particularly the high excitation temperature of the H2 infrared lines. We find that the temperature derived from H2 observations, even of a single knot, will depend very strongly on the observed transitions, with much higher temperatures derived from excited levels. We also proposed that the separation between the Hα and [Nii] peak emission observed in the images of CKs may be an effect of the distance of the knot from the star, since for knots farther from the central star the [Nii] line is produced closer to the border of the CK than Hα. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.