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

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.

Accepted by MNRAS. The paper contains 12 pages, 9 figures, and 3 tables

References in corpus (8)

• Trigonometric Parallaxes of Central Stars of Planetary Nebulae
• Knots in the Helix Nebula found in H2
• Infrared Observations of The Helix Planetary Nebula
• Astrometry with the Hubble Space Telescope: Trigonometric Parallaxes of Planetary Nebula Nuclei: NGC 6853, NGC 7293, Abell 31, and DeHt 5
• Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations
• VLT / Infrared Integral Field Spectrometer Observations of Molecular Hydrogen Lines in the Knots in the Planetary Nebula NGC 7293 (the Helix Nebula)
• Merged ionization/dissociation fronts in planetary nebulae
• H2 infrared line emission from the ionized region of planetary nebulae

Cited by in corpus (5)

• The Herschel Planetary Nebula Survey (HerPlaNS) - a comprehensive dusty photoionization model of NGC6781
• H${}_2$ emission in the low-ionisation structures of the Planetary Nebulae NGC 7009 and NGC 6543
• Spitzer mid-infrared spectroscopic observations of planetary nebulae
• OH${}^{+}$ emission from cometary knots in planetary nebulae
• The Effect of the Slit Configuration on the H${}_2$ 1-0~S(1) to Br$\gamma$ Line Ratio of Spatially Resolved Planetary Nebulae