Mid-infrared spectroscopy and near- and mid-infrared photometry have been obtained for four carbon-rich cool IRAS sources whose IRAS low-resolution spectra are similar to those of proto-planetary nebulae (PPNs) possessing the unusual 21 microns emission feature. The 21 microns feature is detected in all four sources, nearly doubling the number of known 21 microns emitters. The energy distributions of these objects show the characteristic "double-peaked" structure, confirming that they are PPNs. Broad features near 7.7 and 11.3 microns similar to the unidentified infrared features (UIRs) seen in carbon-rich planetary nebulae (PNs) are also detected, suggesting a chemical link between these PPNs and PNs. The presence of the 21 microns feature appears to be correlated with the presence of unusually strong 3.4-3.5 microns features. Possible scenarios for the evolution of these emission features in the PPN phase are discussed.
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Kwok et al. (1995) studied this question.