Modelling the Warm H2 Infrared Emission of the Helix Nebula Cometary Knots

TL;DR

This study models the infrared H2 emission in Helix Nebula's cometary knots, revealing that emission in the interface H+/H0 region can explain observed high excitation temperatures and emission characteristics.

Contribution

It introduces a new modeling approach using the Aangaba code to explain H2 emission in cometary knots, focusing on the interface region, which previous PDR models could not fully account for.

Findings

H2 emission can originate in the H+/H0 interface region.

The observed excitation temperature depends on the transition levels.

The separation of emission peaks relates to the knot's distance from the star.

Abstract

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 ionised 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 ionised gas. Most of the H2 emission of the CKs seems to be produced in a thin layer between the ionised diffuse gas and the neutral material of the knot, in a mini photodissociation region (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 photoionisation code \textsc{Aangaba} to study the H2 emission of the CKs, particularly that produced in the interface H^+/H^0 of the knot, where a significant fraction of the H2 1-0S(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 H_alpha 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 H_alpha.