Origin of broad interstellar feature at 1.6 µm⁻¹

Abstract

DIFFUSE interstellar absorption bands at 4,430, 4,760, 4,890 and 6,180 Å have been assigned to crystal-field transitions of octahedrally and tetrahedrally bonded ferric ions in an iron oxide^1–3. The optical absorption spectra of terrestrial iron bearing minerals^4–6 often comprise, in addition to the crystal-field bands, a broad absorption (half-width 3,000 cm⁻¹) in the range 14,000–18,500 cm⁻¹ (715–540 nm) marking Fe²⁺3dt₂ → Fe³⁺3dt₂ intervalence charge transfer, the cations being located at the centres of adjacent edge-sharing octahedra. Here, I suggest that a discontinuity at ∼ 1.6 µm⁻¹, which I see as a minor feature on the mean extinction curve⁷ for several reddened stars, represents Fe²⁺→Fe³⁺ charge transfer absorption in interstellar grains. A shallow emission feature observed at ∼ 1.8 µm⁻¹ in the reddening curve for another group of stars has been interpreted⁸ as a dip lying between two absorptions centred at ∼ 1.5 and ∼ 2.4 µm⁻¹.