Infrared Imaging and Spectroscopy of the Helix with ISOCAM

We report infrared images of the Helix Nebula centered at 6.9 μm (LW2 filter) and 15 μm (LW3 filter) obtained with ISOCAM on board the Infrared Space Observatory ( ISO ). Three fields were also measured using the ISOCAM circular variable filter (CVF). The CVF data show that the 5-16.6 μm spectrum is dominated by the pure ( v =0-0) rotational lines of molecular hydrogen from the S (7) to the S (2) transitions. The strong S (5) H 2 line accounts for most of the emission detected in the LW2 filter. The only atomic lines detected are [Ne II] 12.81 μm and [Ar III] 8.99 μm, which are weak, and [Ne III] 15.55 μm, which is strong and accounts for most of the emission in the LW3 filter. No emission bands or continuum of small dust particles are detected despite the carbon richness of the Helix Nebula. The H 2 emission traces the individual cometary globules of the molecular envelope of the nebula, whereas the [Ne III] emission is distributed along this envelope toward the inner regions of the ionized cavity. The intensities of H 2 rotational lines are accurately predicted using a rotational temperature of 900±50 K and column densities of ~3 × 10 18 cm -2 . The total luminosity in the H 2 lines ~4 L ☉ (6% of the star luminosity) is much higher than predicted for photodissociation regions. The significant absence of mid-infrared dust features indicates that in this evolved planetary nebula, the molecular-sized dust particles might have been destroyed by the exposure to the radiation field from the central hot star.