INFRARED IMAGING AND SPECTROSCOPY OF THE HELIX WITH ISOCAM

We report infrared images of the Helix Nebula centered at 6.9 mu m (LW 3 filter) and 15 mu m (LW3 filter) obtained with ISOCAM on board the I nfrared Space Observatory (ISO). Three fields were also measured using the ISOCAM circular variable filter (CVF). The CVF data show that the 5-16.6 mu m spectrum is dominated by the pure (v = 0-0) rotational li nes of molecular hydrogen from the S(7) to the S(2) transitions. The s trong 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 mu m and [Ar III] 8.99 mu m, which are weak, and [Ne III] 15.55 mu m, which is strong and accounts for most of the emission in the LW3 filter. No emi ssion bands or continuum of small dust particles are detected despite the carbon richness of the Helix Nebula. The H-2 emission traces the i ndividual cometary globules of the molecular envelope of the nebula, w hereas the [Ne III] emission is distributed along this envelope toward the inner regions of the ionized cavity. The intensities of H-2 rotat ional lines are accurately predicted using a rotational temperature of 900 +/- 50 K and column densities of similar to 3 x 10(18) cm(-2). Th e total luminosity in the H-2 lines similar to 4 L-circle dot (6% of t he star luminosity) is much higher than predicted for photodissociatio n regions. The significant absence of mid-infrared dust. features indi cates that in this evolved planetary nebula, the molecular-sized dust particles might have been destroyed by the exposure to the radiation f ield from the central hot star.