The role of polycyclic aromatic hydrocarbons in ultraviolet extinction. I. Probing small molecular polycyclic aromatic hydrocarbons

We have obtained new Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra to search for structure in the ultraviolet interstellar extinction curve, with particular emphasis on a search for absorption features produced by polycyclic aromatic hydrocarbons (PAHs). The presence of these molecules in the interstellar medium has been postulated to explain the infrared emission features seen in the 3 - 13 mum spectra of numerous sources. Ultraviolet (UV) spectra are uniquely capable of identifying specific PAH molecules. We obtained high signal-to-noise ratio UV spectra of stars that are significantly more reddened than those observed in previous studies. These data put limits on the role of small ( 30 - 50 carbon atoms) PAHs in UV extinction and call for further observations to probe the role of larger PAHs. PAHs are of importance because of their ubiquity and high abundance inferred from the infrared data, and also because they may link the molecular and dust phases of the interstellar medium. A presence or absence of UV absorption bands due to PAHs could be a definitive test of this hypothesis. We should be able to detect a 20 Angstrom wide feature down to a 3 sigma limit of similar to 0.02 A(V). No such absorption features are seen other than the well-known 2175 Angstrom bump.