Dust extinction-curve variation in the translucent interstellar medium is driven by PAH growth

The first all-sky, high-resolution, 3D map of the optical extinction curve of the Milky Way (Zhang & Green 2024) 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 anti-correlation 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-two stronger 2175˚A 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.