MIDINFRARED SPECTROSCOPY OF CARBON-RICH POST-AGB OBJECTS AND DETECTION OF THE PAH MOLECULE CHRYSENE

We present 10 and 20-mu m spectroscopic observations of a sample of F- and C-type carbon-rich post-AGE objects, three of which were known to exhibit the unidentified emission feature at 21 mu m. We also find th e F3I post-AGE object SAO 163075 to exhibit a (weak) 21-mu m emission feature. We additionally obtained a 10-mu m spectrum of IRAS 05341+085 2, which has been reported to have a possible 21-mu m emission band in its IRAS LRS spectrum, and obtained new 10 and 20-mu m spectra of the carbon-rich bipolar post-AGE sources GL 2688 and GL 618, the extreme carbon star GL 3068, and the planetary nebulae IRAS 21282+5050 and He 2-447, in order to study the evolution of C-rich dust from the early p ost-AGE through to the planetary nebula (PN) phases. The 7.5-13-mu m s pectra of the 21-mu m band objects exhibit broad plateau emission, sho rtwards of 9 mu m and from 10-13 mu m, superposed upon which, in addit ion to the well-known UIR-band at 11.3 mu m, are several new features, at 7.9, 8.2, 10.6, 11.5 and 12.2 mu m, differing from those observed in standard UIR-band spectra. An excellent match is found between the wavelengths of these new features and those of bands in the spectrum o f chrysene (C18H12), one of the simplest PAH molecules. The absence of the new features in the spectra of earlier spectral-type post-AGE obj ects and PN is consistent with the expected complete dehydrogenation o f any PAH molecules having less than 20-25 carbon atoms when exposed t o the ultraviolet radiation fields of stars with spectral types earlie r than F. Chrysene is not responsible for the 21-mu m emission bands o bserved in the spectra of the cool post-AGE objects. Possible identifi cations for the 21-mu m band in terms of highly hydrogenated 2-dimensi onal PAH molecules or 3-dimensional fulleranes (hydrogenated fullerene s) are discussed, The mid-infrared spectrum of GL 2688 is largely feat ureless, apart from two broad weak emission features between 9.5-10.5 and 10.5-12.2 mu m. The profiles of the broad 10-13-mu m absorption fe atures in the spectra of the extreme carbon star GL 3068 and the C-ric h bipolar post-AGE object GL 618 are compared. For GL 3068 the profile shape and the wavelength of peak absorption, near 11 mu m, are consis tent with absorption by silicon carbide particles. However, the absorp tion observed towards GL 618 is considerably broader and peaks at 12 m u m. Its profile is very similar to that of the 10-13-mu m emission pl ateau observed in the spectra of the 21-mu m band objects, suggesting that it arises from absorption by a large column of highly-hydrogenate d PAH-type species.