Modeling the infrared interstellar extinction

• The flat mid-IR extinction could not be explained by the standard MRN silicate–graphite dust model.
• We aim to explain the flat mid-IR extinction.
• We consider amorphous silicate, graphite, amorphous carbon and iron dust.
• The flat IR extinction is best explained in terms of micrometer-sized amorphous carbon dust.

How dust scatters and absorbs starlight in the interstellar medium (ISM) contains important clues about the size and composition of interstellar dust. While the ultraviolet (UV) and visible interstellar extinction is well studied and can be closely fitted in terms of various dust mixtures (e.g., the silicate–graphite mixture), the infrared (IR) extinction is not well understood, particularly, the mid-IR extinction in the 3–8 μm wavelength range is rather flat (or “gray”) and is inconsistent with the standard Mathis, Rumpl, & Nordsieck (MRN) silicate–graphite dust model. We attempt to reproduce the flat IR extinction by exploring various dust sizes and species, including amorphous silicate, graphite, amorphous carbon and iron. We find that the flat IR extinction is best explained in terms of micrometer-sized amorphous carbon dust which consumes ~60~60 carbon atoms per million hydrogen atoms (i.e., C/H ≈60 ppm). To account for the observed UV/visible and near-IR extinction, the silicate–graphite model requires Si/H≈34 ppm and C/H≈292 ppm. We conclude that the extinction from the UV to the mid-IR could be closely reproduced by a mixture of submicrometer-sized amorphous silicate dust, submicrometer-sized graphitic dust, and micrometer-sized amorphous carbon dust, at the expense of excess C available in the ISM (i.e., this model requires a solid-phase C abundance of C/H ≈ 352 ppm, considerably exceeding what could be available in the ISM).