Tentative identification of pyrene as a polycyclic aromatic molecule in UV spectra of comet P/Halley: An emission from 368 to 384 nm
Emission spectra of comet P/Halley in the 275- 710 nm wavelength range were obtained using a spectrometer mounted on the Vega 2 spacecraft, which encountered the comet on March 9, 1986. The spectra, after the removal of the dust-scattered solar continuum, show the presence of a broad-band emissive feature between 340 and 390 nm with three peaks at 371, 376 and 382 nm. Near the nucleus, the intensity increase illustrates that the molecules responsible for the emission are most likely of the parent type. Our cometary spectra were compared with UV laboratory spectra of polycyclic aromatic hydrocarbons having 4 benzenoid rings. A laser-induced fluorescence experiment conducted in the laboratory recorded the dispersed emission spectrum of pyrene under jet-cooled conditions. Moreels et al. (Astron. Astrophys. 284 (1994) 643) demonstrated that phenanthrene was a possible candidate for the four bands at 347, 356, 364 and 374 nm. The comparison between the cometary and laboratory spectra suggests the possible presence of another PAH, probably pyrene, in Halley's comet. This new suggestion illustrates the link between cometary and interstellar matter. These observations are coherent with the detection of the cometary IR band at 3.28 μm assigned to an X-CH organic compound.