4 NEWLY IDENTIFIED SOURCES WITH 21 MICRON EMISSION

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 mu m emission feature. The 21 mu m feature is detected in all four sources, nearly doubling the number of known 21 mu m 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 mu m similar to the unidentified infrared features (UTRs) seen in carbon-rich planetary nebulae (PNs) are also detected, suggesting a chemical link between these PPNs and PNs. The presence of the 21 mu m feature appears to lie correlated with the presence of unusually strong 3.4-3.5 mu m features. Possible scenarios for the evolution of these emission features in the PPN phase are discussed.