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Astron. Astrophys. 315, L373-L376 (1996)

The shape and strength of circumstellar PAH emission bands*

F.J. Molster1, M.E. van den Ancker1, A.G.G.M. Tielens2, L.B.F.M. Waters1,3, D.A. Beintema3, C. Waelkens4, T. de Jong3,1, Th. de Graauw3, K. Justtanont3, I. Yamamura3, B. Vandenbussche4,5, and A. Heras5

1 Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, USA
3 SRON Laboratory for Space Research Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
4 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium
5 ESA Villafranca, P.O. Box 50727, E-28080 Madrid, Spain

Received 29 July 1996 / Accepted 28 August 1996

Abstract. We discuss the shape and strength of the emission bands observed in Short Wavelength Spectrometer (SWS) spectra of three carbon-rich evolved stars (Beintema et al. 1996a). The emission bands, due to stretching and bending modes of C-C and C-H bonds in Polycyclic Aromatic Hydrocarbons (PAHs), show large difference in strength and shape, depending on the excitation temperature and size distribution. We find that HR 4049 shows remarkable structure in the major bands (3.3, 6.2, '7.7', 8.6 and 11.3 img1.gifm) and probably has small, ionized, hydrogen-rich and highly excited PAHs. In contrast, the Planetary Nebulae IRAS21282+5050 and NGC 7027 show a larger abundance of neutral PAHs and plateau emission near 7-8 and 11-12  img1.gifm, probably due to larger molecules. The population of PAHs in HR 4049 probably has a different evolutionary history than those in the planetary nebulae.

Key words: stars: AGB and post-AGB - stars: mass loss - dust - planetary nebulae - infrared: stars

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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