Discovery of a 6.4 micron dust feature in hydrogen-poor planetary nebulae

Hydrogen-poor planetary nebulae (PNe) Abell 58, Abell 78, and IRAS 1515425258 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 mm dust feature is present instead in the IR spectra of Abell 78 and IRAS 1515425258. We identify this feature as an aromatic C-C stretch feature produced in carbonaceous grains with little or no hydrogen. The 6.4 mm feature is absent in the IR spectrum of the hydrogen-poor material in PN A58, which was ejected nearly 90 yr ago. Our observations of all three PNe are consistent with the hypothesis that the 6.4 μm C-C stretch feature is produced by small, H-free carbonaceous grains, transiently heated to high temperatures by energetic stellar photons. The 6.4 μm feature is not detected in A58 because of the presence of strong IR continuum produced by large, hot grains located close to its central star.