F.J. Molster, M.E. van den Ancker, A.G.G.M. Tielens, L.B.F.M. Waters, D.A. Beintema, C. Waelkens, T. de Jong, Th. de Graauw, K. Justtanont, I. Yamamura, B. Vandenbussche, A. Heras
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
m) 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 m, probably due to larger
molecules. The population of PAHs in HR 4049 probably has a different
evolutionary history than those in the planetary nebulae.
Keywords:
Stars: AGB and post-AGB - Stars: mass loss -
Dust - Planetary Nebulae - Infrared: stars