Neutral and ionized polycyclic aromatic hydrocarbons, diffuse interstellar bands and the ultraviolet extinction curve

Abstract

Neutral naphthalene (C₁₀H₈), phenanthrene (C₁₄H₁₀) and pyrene (C₁₆H₁₀) absorb strongly in the ultraviolet region and may contribute to the extinction curve. High abundances are required to produce detectable structures. The cations of these polycyclic aromatic hydrocarbons (PAHs) absorb in the visible. C₁₀H⁺ ₈ has 13 discrete absorption bands which fall between 6800 and 4500 Å. The strongest band at 6741 Å falls close to the weak 6742 Å diffuse interstellar band (DIB). Five other weaker bands also match DIBs. The possibility that C₁₀H⁺ ₈ is responsible for some of the DIBs can be tested by searching for new DIBs at 6520 and 6151 Å, other strong naphthalene cation band positions. If C₁₀H⁺ ₈ is indeed responsible for the 6742 Å feature, it accounts for 0.3% of the cosmic carbon. The spectrum of C₁₆H⁺ ₁₀ is dominated by a strong band at 4435 Å in an Ar matrix and 4395 Å in Ne, wavelengths which fall very close to the strongest DIB at 4430 Å. If C₁₆H⁺ ₁₀, or a closely related pyrene-like ion, is indeed responsible for the 4430 Å feature, it accounts for 0.2% of the cosmic carbon. An intense, very broad UV-to-visible continuum is reported which is associated with both ions and could explain how PAHs convert interstellar UV and visible readiation into IR radiation.