THE VISIBLE AND ULTRAVIOLET-ABSORPTION OF LARGE POLYCYCLIC AROMATIC-HYDROCARBONS

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-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% of the cosmic carbon. Adopting this abundance, we find that large PAHs only contribute a smooth continuum at the bottom of the 2200 angstrom 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.