DETECTION OF A POLYCYCLIC AROMATIC MOLECULE IN COMET-P HALLEY/

New spectroscopic data obtained with the three-channel spectrometer of Vega 2 show, after subtracting the dust-scattered solar continuum, th at a broad band emissive feature between 342 and 375 nm progressively arises. Four bands at 347, 356, 364 and 374 nm can be identified in th is feature. The 1/p type intensity increase (p: projected distance bet ween nucleus and line of sight) shows that the molecules responsible f or the emission are most probably of the parent-type. A search for a m olecule emitting fluorescence bands between 342 and 375 nm showed that phenanthrene was a possible candidate. A laser-induced fluorescence e xperiment was conducted in the laboratory to record the dispersed emis sion spectrum of phenanthrene under jet-cooled conditions. An excellen t agreement was obtained between the cometary and laboratory spectra, for both the wavelength positions of the main peaks and their relative intensities. Present observations are coherent with the detection by IKS of an IR band at 3.28 mum assigned to an X-CH organic compound. Th e identification of a PAH in Halley's comet emphasizes the similaritie s between the material released by comets and diffuse interstellar neb ulae. It adds a strong argument in favor of the fact that comets would be constituted of interstellar material.