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Title: What carrier produces the 2175 Angstrom extinction bump?
Plain: Interstellar dust absorbs ultraviolet light most strongly near a wavelength of 2175 Angstrom. The bump's position barely changes from place to place while its width does, and no material reproduces both facts.
Precise: Identify the carrier of the 2175 Angstrom extinction feature (candidates: graphitic carbon grains, PAH mixtures, hydrogenated carbon, carbon onions) and explain why its central wavelength is constant to about 1 percent across sightlines while its width varies by about 20 percent or more.
Settled by: Laboratory or ab initio optical spectra of a carrier population reproducing the fixed peak, variable width and observed strength per hydrogen atom.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: JWST detected the bump in a galaxy at $z \sim 6.7$ (Witstok et al., Nature, 2023), requiring the carrier to form within about 1 Gyr.
Posed since: 1965