Polycyclic aromatic hydrocarbon ions and the diffuse interstellar bands.

Abstract

Neutral naphthalene (C10H8), phenanthrene (C14H10), and pyrene (C16H10) absorb strongly in the ultraviolet and may contribute to the extinction curve. High abundances are required to produce detectable structures. The cations of these PAHs absorb in the visible. C10H8+ has 12 discrete absorption bands which fall between 6800 and 5000 angstroms. The strongest band at 6741 angstroms falls close to the weak 6742 angstroms diffuse interstellar band (DIB). Five other weaker bands also match DIBs. The possibility that C10H8+ is responsible for some of the DIBs can be tested by searching for new DIBs at 6520, 6151, and 5965 angstroms, other moderately strong naphthalene cation band positions. If C10H8+ is indeed responsible for the 6742 angstroms feature, it accounts for 0.3% of the cosmic carbon. The spectrum of C16H10+ is dominated by a strong band at 4435 angstroms in an Ar matrix and 4395 angstroms in a Ne matrix, a position which falls very close to the strongest DIB, that at 4430 angstroms. If C16H10+, or a closely related pyrene-like ion is indeed responsible for the 4430 angstroms feature, it accounts for 0.2% of the cosmic carbon. We also report an intense, very broad UV-to-visible continuum which is associated with both ions and could explain how PAHs convert interstellar UV and visible radiation into IR.