The following document lists the file abstract/PCOX_ZZPN.abs
from catalogue VI/111.
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ISOCAM CVF observations on the Helix nebula, the closest planetary nebula (PN), have shown the surprising result that the usual broad emission mid-infrared dust bands are absent indicating that molecular-sized grains (such as PAH) are not present in the carbon-rich envelope of this fully evolved PN. The spectrum is instead dominated by the pure rotational lines of molecular hydrogen (Cox et al. 1998). A second case appears to the Dumbbell nebula where the ISOCAM imaging results point towards a similar behaviour, i.e, no dust bands and molecular hydrogen lines dominating the mid-infrared emission. In both cases, these nebulae will not enrich the interstellar medium in PAH or small grains. It is thus of interest to study other evolved carbon-rich PNe and check if this result is general, in which case an efficient process of producing small grains must be active in the interstellar medium to account for their ubiquitous presence in space. We thus propose to follow-up these ISO results to further investigate the properties of the dust grains in the Helix (at long wavelengths in order to derive the size distribution of the dust population in this nebula) and in the Dumbbell (to obtain ISOCAM CVF data). Furthermore, we propose to investigate additional PNe which are less evolved than the Helix and the Dumbbell in order to study the evolution of the spectral properties of dust particles in the mid-infrared wavelength range using the ISOCAM CVF and also the SWS. The proposed observations are fundamental for understanding the evolution of the dust properties in the envelopes of planetary nebulae and to understand what is the nature of the grains which will ultimately enrich the interstellar medium. This program is a "sollicited" ISO program.