UNUSUAL 3 MICRON EMISSION FEATURES IN 3 PROTO PLANETARY-NEBULAE

Medium-resolution 3-4-mu-m spectra have been obtained of three objects, IRAS 04296 + 3429, IRAS 22272 + 5435, and CRL 2688, which are in transition between the asymptotic red giant branch and the planetary nebula phase. All three show unusual 3-mu-m emission features. The two IRAS objects, already unusual in having 21-mu-m features, have remarkably strong 3.4-3.5-mu-m emission relative to the usually dominant 3.3-mu-m feature, with the spectrum of IRAS 04296 + 3429 being nearly identical to that of IRAS 05341 + 0852 (previously published). CRL 2688's spectrum bears some resemblance to those of the IRAS objects, but its 3.4-3.5-mu-m emission is not as strong. The relationships between the various emission features are discussed. CRL 2688 may be completing a transition between the "21-mu-m phase" and the "normal phase," which is associated with dominant 3.3-mu-m emission, weak 3.4-3.6-mu-m features and plateau, and a lack of 21-mu-m emission.