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arXiv · 2410.23171

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

Xiangyu Zhang·Brandon S. Hensley·Gregory M. Green
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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.

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Xiangyu Zhang, Brandon S. Hensley, Gregory M. Green. 2024-10-30. Dust extinction-curve variation in the translucent interstellar medium is driven by PAH growth. https://arxiv.org/abs/2410.23171

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