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B.J. Hrivnak (Valparaiso U.), K. Volk, S. Kwok (U. Calgary)
We have used the Infrared Space Observatory (ISO) to obtain spectra of a sample of proto-planetary nebulae (PPNs). These PPNs have a wide range in visible brightness, V = 8 to 22 mag, but all possess strong infrared excesses due to circumstellar dust. The spectra were obtained in the wavelength range 2.4 to 45 \mum with the Short Wavelength Spectrometer, at an effective resolution of ~250.
The objects in the present study all displayed spectral characteristics which indicate that they are carbon-rich. PAH emission features are seen at 3.3, 6.2, 6.9, 7.8, and 11.3 \mum. In addition, most possess the broad "30 \mum" emission feature seen in carbon-rich AGB stars, PPNs, and PNs, and some possess the "21 \mum" emission feature seen only in PPNs. Several of the 30 \mum sources and one of the 21 \mum sources are new detections; others are confirmations of previous observations made by IRAS, the Kuiper Airborne Observatory, or ground-based spectral observations.
We have modeled their spectral energy distributions using a radiative transfer code and assuming that the underlying infrared continuum is due to emission from amorphous carbon grains. We then empirically fitted the 30 \mum and 21 \mum features. Circumstellar shell sizes, mass loss rates, and dust temperatures (~200 K) were determined. We calculated the strengths of the various spectral features, and find that 10-20 % of the infrared flux originates from the 30 \mum feature.
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