A search for PAHs in the ISM: High-resolution UV observations confronted with laboratory spectra

Details A search for PAHs in the ISM: High-resolution UV observations confronted with laboratory spectra A search for PAHs in the ISM: High-resolution UV observations confronted with laboratory spectra

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.320r&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

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Interstellar polycyclic aromatic hydrocarbon (PAH) molecules are expected to contribute to physical and chemical processes that take place in the interstellar medium (ISM). In order to evaluate the magnitude of these processes, the abundance of each PAH species has to be determined. To date, the only direct evidence of interstellar PAH molecules consists of the aromatic infrared bands (AIBs). These features, however, correspond to the combined and unresolved contributions of various populations of PAH species. Consequently, their analysis results in abundances that refer to average PAH populations. On the other hand, under conditions relevant to the ISM, individual PAH species can be identified by their electronic spectra. Considering neutral PAHs in their electronic ground state, the lowest-energy electronic transition shifts from the UV region to the visible as the size of the molecules increases. Still, for large PAHs, the strongest transitions are often found in the UV. We started a search for electronic bands of interstellar PAHs in the UV region with the goal of estimating their column densities and fractional abundances. For this purpose, we recently carried out high-resolution observations in the 3050 -- 3850 Å region with the ultraviolet and visual echelle spectrograph (UVES) at the VLT. Observational spectra were obtained with a high signal-to-noise ratio toward five reddened supergiants. The velocity scales were fixed by using the absorption lines of detected diatomic molecules, namely OH^+, NH, CH, CH^+, and CN. In our laboratory, the absorption spectra of several PAHs molecules were measured under conditions relevant to the ISM, that is in the gas phase at low density and low temperature. The spectra of anthracene, phenanthrene, pyrene, 2,3-benzofluorene, and benzo[g,h,i]perylene were calibrated in terms of absorption cross section by comparing them with quantitative spectra measured in solutions. Spectra taken in cryogenic matrices were used in an intermediate step. We did not find PAH bands in the UVES spectra. Nonetheless, by comparing them with the laboratory measurements, we estimated upper limits for the column densities and fractional abundances for each of the PAHs in our set. Our results suggest either a low abundance of neutral PAHs in translucent molecular clouds or the presence of a very varied population.

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