2011MNRAS.416..790A

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 H₂1–0 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 H₂observations, 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 [N II] 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 [N II] line is produced closer to the border of the CK than Hα.