The visible and ultraviolet absorption of large polycyclic aromatic hydrocarbons

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Absorption Spectra, Hydrocarbons, Interstellar Extinction, Ultraviolet Spectra, Visible Spectrum, Absorption Cross Sections, Abundance, Graphite, Molecular Spectra

Scientific paper

To assess the contribution of polycyclic aromatic hydrocarbons (PAH) to the interstellar extinction, we have determined the absorption cross-section of large PAHs in the visible-UV range from graphite optical constants. It is dominated by 2 strong features involving pi-electrons at low energies (5.6 eV) and sigma micron electrons at higher energies (16 eV). From the interstellar near-IR emission and extinction curve, constraints on the PAH abundance are derived using this cross-section: we estimate that PAHs bear about 10 percent of the cosmic carbon. Adopting this abundance, we find that large PAHs only contribute a smooth continuum at the bottom of the 2200 A interstellar feature. On the other hand, we argue that large PAHs dominate the far-UV non-linear extinction. Other small grain populations are needed to account for the far-UV linear rise.

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