Unusual 3 micron emission features in three proto-planetary nebulae

Abstract

Medium-resolution 3-4 μm 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 μm emission features. The two IRAS objects, already unusual in having 21 μm features, have remarkably strong 3.4-3.5 μm emission relative to the usually dominant 3.3 μm 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 μm emission is not as strong. The relationships between the various emission features are discussed. CRL 2688 may be completing a transition between the "21 μm phase" and the "normal phase," which is associated with dominant 3.3 μm emission, weak 3.4-3.6 μm features and plateau, and a lack of 21 μm emission.