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

TL;DR

This study links variations in the interstellar extinction curve to PAH growth in the translucent ISM, supported by high-resolution 3D mapping and infrared data, suggesting PAH accretion influences dust properties.

Contribution

It introduces a novel explanation that PAH growth via gas-phase accretion causes observed extinction curve variations in the Milky Way's translucent ISM.

Findings

Strong anti-correlation between $R(V)$ and PAH abundance $q_{PAH}$.

PAH growth by a factor of ~2 between $A_V$ of 1 and 3.

PAH growth accounts for the observed change in the 2175 Angstrom feature.

Abstract

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 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 PAH abundance -- parameterized by $q_{\rm PAH}$ -- derived from infrared emission. The range of observed $q_{\rm PAH}$ indicates PAH growth by a factor of $\sim$2 between $A_V \simeq 1$ and 3. This implies a factor-of-two stronger 2175 Angstrom 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.