Dust-extinction-curve Variation in the Translucent Interstellar Medium Is Driven by Polycyclic Aromatic Hydrocarbon Growth

The first all-sky, high-resolution, 3D map of the optical extinction curve of the Milky Way revealed an unexpected steepening of the extinction curve in the moderate-density, “translucent” interstellar medium (ISM). We argue that this trend is driven by growth of the total mass of polycyclic aromatic hydrocarbons (PAHs) through gas-phase accretion. We find a strong anticorrelation between the slope of the optical extinction curve—parameterized by R(V)—and maps of the PAH mass fraction (relative to the total dust mass)—parameterized by qPAH—derived from infrared emission. The range of observed qPAH indicates PAH growth by a factor of ∼2 between AV ≃ 1 and 3. This implies a factor-of-2 stronger 2175 Å feature, which is sufficient to lower R(V) by the observed amount. This level of PAH growth is possible given rapid accretion timescales and the depletion of carbon in the translucent ISM. Spectral observations by JWST would provide a definitive test of this proposed explanation of R(V) variation.