Infrared Imaging and Spectroscopy of the Helix with ISOCAM

Abstract

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) H2 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.