ISO Spectroscopy of a Sample of Carbon-Rich Proto-Planetary Nebulae

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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 mu m 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 mu m. In addition, most possess the broad "30 mu m" emission feature seen in carbon-rich AGB stars, PPNs, and PNs, and some possess the "21 mu m" emission feature seen only in PPNs. Several of the 30 mu m sources and one of the 21 mu m 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 mu m and 21 mu m 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 mu m feature.

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