Stellar Cartography

Dust Extinction-curve Variation In The Translucent Interstellar Medium Is Driven By PAH Growth

By Keith Cowing

Status Report

astro-ph.GA

October 31, 2024

Filed under astro-ph.GA, astrochemistry, astronomy, Interstellar, interstellar medium, JWST, Milky Way, PAH, Polycyclic Aromatic Hydrocarbons, Spectroscopy, Stellar Cartography

Dust Extinction-curve Variation In The Translucent Interstellar Medium Is Driven By PAH Growth — Comparison of 2D distribution of A(V), R(V), q_PAH for dust clouds. For A(V) and R(V) maps, we select high-quality stars from Zhang & Green (2024), and remove lines of sight where only partial extinction is covered by XP. For qPAH map, we remove lines of sight where the q_PAH map is of low quality (χ 2 /DOF > 3 or q_PAH/σ(q_PAH) < 3), or that cross multiple clouds (Equation 1). The binning method and selection of stars in the 2D maps are described in Appendix A. Despite different baselines and finer structures in different maps, all clouds tend to have lower R(V ) at high-q_PAH regions, as indicated quantitatively on the per-star basis in Figure 5 — astro-ph.GA

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_PAH — derived from infrared emission. The range of observed q_PAH indicates PAH growth by a factor of ∼2 between AV≃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.

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

Comments: 11 pages, 6 figures, submitted on October 30, 2024
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2410.23171 [astro-ph.GA] (or arXiv:2410.23171v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2410.23171
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Submission history
From: Xiangyu Zhang
[v1] Wed, 30 Oct 2024 16:22:23 UTC (2,821 KB)
https://arxiv.org/abs/2410.23171

Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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