Abstract
After subtracting the intense dust-scattered continuum from the original spectra transmitted by the Vega 2 three-channel spectrometer, a broad-band emission emerges in the 342–375 nm spectral range when the cometocentric projected distance p is smaller than 5000 km. This newly detected emission varies as p −1, which implies that the involved molecule(s) has a parent-type behavior. The emission band presents four peaks at 347, 356, 364 and 373 nm. It is tentatively identified as being due to phenanthrene, a three-cycle aromatic condensed hydrocarbon. A determination of the gQ product, where g is the fluorescence quantum efficiency and Q the production rate gives gQ = 1.2 × 10 25. If g = 0.012, it comes Q = 1 × 10 27 s −1. The detection of phenanthrene in Halley's inner coma is an important argument in favor of a similarity of composition between cometary material and interstellar matter. It supports the hypothesis that comets have kept trace of the interstellar composition through the solar system formation epoch.
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