2-45 micron infrared spectroscopy of carbon-rich proto-planetary nebulae

Infrared Space Observatory (ISO) 2-45 mu m observations of seven proto-plan etary nebulae (PPNs) and two other carbon-rich objects are presented. The u nidentified emission features at 21 and 30 mu m are detected in six sources , including four new detections of the 30 mu m feature. This previously unr esolved 30 mu m feature is now resolved and found to consist of a broad fea ture peaking at 27.2 mu m (the "30 mu m" feature) and a narrower feature at 25.5 mu m (the "26 mu m" feature). This new 26 mu m feature is detected in eight sources and is particularly strong in IRAS Z02229 + 6208 and 16594- 4656. The unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, a nd 11.3 mu m, which are commonly observed in planetary nebulae and H II reg ions, are also seen in these PPNs. However, their strengths relative to the continuum plateaus at 8 and 12 mu m are weaker than in planetary nebulae. The 6.9 mu m feature, seen almost exclusively in PPNs, is strong. New milli meter CO and HCN observations were made; they support the carbon-rich natur e of the objects and yield the expansion velocities of the gaseous envelope s. The spectral energy distributions of these PPNs were fitted with a radia tive-transfer model, taking into account the emission features at 21, 26, a nd 30 mu m. A significant fraction of the total energy output is emitted in these features: as high as 20% in the 30 mu m feature and 8% in the 21 mu m feature. The fact that so much energy is carried in these features sugges ts that the material responsible for these features must be made of abundan t elements and most likely involves carbon. SiS2 appears to be ruled out as the emitter of the 21 mu m feature due to the absence of a predicted compa nion feature.