2008A&A...492..245C

Polycyclic aromatic hydrocarbons (PAHs) have been detected in many interstellar medium (ISM) sources. They are considered among the most abundant organic compounds in these environments. We aim at identifying the carriers of the near UV fluorescence bands that appear in the spectra of 1P/Halley's inner coma. Near UV spectra of 1P/Halley were recorded on March 9, 1986 by the three-channel spectrometer onboard the Vega2 spacecraft at projected distances of between 421 and 932 km. We compare these data to laboratory spectra obtained under laser-induced fluorescence conditions in a jet-cooled molecular beam. The cometary spectral features are found to be consistent with the laboratory fluorescence spectrum of anthracene. Four main peaks coincide at 363, 367.5, 373 and 382.5nm. We then report the identification of anthracene, a three-ring PAH in the inner coma of 1P/Halley at projected distances of less than thousand km. We derive an abundance relative to water of 5x10^–5 to 1x10^–4 for this molecule. This new detection adds to the previously reported UV identifications of phenanthrene and pyrene in the innermost coma of 1/P Halley. Recently, three small PAHs, namely naphtalene, phenanthrene and pyrene, were identified in dust grains collected by the Stardust probe in the environment of Comet 81P/Wild2. These findings, together with the present identification of anthracene in 1P/Halley's near UV spectra, confirm that similarities exist between the composition of comets and that of the ISM.