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J. Clairemidi, C. Reylé-Laffont, G. Moreels (Observatoire de Besançon), P. Brechignac (Laboratoire de Photophysique Moleculaire, Univ. Paris Sud)
Emission spectra of comet P/Halley in the 360-390 nm wavelength range were obtained by using the three-channel spectrometer carried on board the Vega2 spacecraft which encountered the comet on March 9, 1986. After subtraction of the dust-scattered solar continuum, the spectra show the presence of new spectral features: a broad band with peaks at 363, 367, 371, 376 and 382 nm.
The 1/p-type intensity increase (p: projected distance between the nucleus and the line-of-sight) suggests that the molecules responsible for the emission are most probably of the parent type. We compared our cometary spectra with UV laboratory spectra of polycyclic aromatic hydrocarbons having 2 and 4 benzenoid rings. A laser-induced fluorescence experiment was conducted in the laboratory to record the dispersed emission spectrum of anthracene and pyrene under jet-cooled conditions. The comparison between the cometary and laboratory spectra suggests the presence of P.A.H.: probably anthracene and pyrene in Halley's comet.
In 1994, we showed that phenanthrene was a possible candidate for the four bands at 347, 356, 364 and 374 nm. This presence adds a strong argument in favour of the fact that comets are constituted of interstellar matter.
We present the results of the comparison between Vega2 spectra and laboratory spectra. The radial intensity at the considered wavelength is shown for the different wavelengths of the P.A.H. and for daughter molecules.