DISCOVERY OF A 6.4 MICRON DUST FEATURE IN HYDROGEN-POOR PLANETARY-NEBULAE

Hydrogen-poor planetary nebulae (PNe) Abell 58, Abell 78, and IRAS 151 54-5258 were observed with the ISOPHOT low-resolution spectrometer on board the Infrared Space Observatory. Polycyclic aromatic hydrocarbon (PAH) features, commonly seen in H II regions, PNe, and in the diffuse interstellar medium, are absent from their infrared (IR) spectra. A 6 .4 mu m dust feature is present instead in the IR spectra of Abell 78 and IRAS 15154-5258. We identify this feature as an aromatic C-C stret ch feature produced in carbonaceous grains with little or no hydrogen. The 6.4 mu m feature is absent in the TR spectrum of the hydrogen-poo r material in PN A58., which was ejected nearly 90 yr ago. Our observa tions of all three PNe are consistent with the hypothesis that the 6.4 mu m C-C stretch feature is produced by small, H-free carbonaceous gr ains, transiently heated to high temperatures by energetic stellar pho tons. The 6.4 mu m feature is not detected in A58 because of the prese nce of strong LR continuum produced by large, hot grains located close to its central star.