Aromatic, aliphatic, and the unidentified 21 micron emission features in proto-planetary nebulae

Aromatic features at 3.3, 6.2, 7.7, 8.6, 11.3 μm are observed in proto-planetary nebulae (PPNe) as well as in PNe and H II regions. Aliphatic features at 3.4 and 6.9 μm are also observed; however, these features are often stronger in PPNe than in PNe. These observations suggest an evolution in the features from simple molecules (C₂H₂) in AGB stars to aliphatics in PPNe to aromatics in PNe. In the same carbon-rich PPNe, a strong, broad, unidentified 21 μm emission feature has been found. We will present recent observations of the aromatic, aliphatic, and 21 μm emission features, along with C₂H₂ (13.7 μm) and a new feature at 15.8 μm, and discuss correlations among them and other properties of these PPNe.