Is R(V) driven by PAHs?

Xiangyu Zhang

Tuesday, Dec. 3rd, 16:20CET

A counter-intuitive anti-correlation between the slope of the extinction curve - parameterized by R(V) - and the dust column density A(V) in translucent ISM is noticed from the full-sky R(V) map of the Milky Way and LMC (Zhang & Green 2024). R(V) only rises in higher-density ISM such as star-forming regions. We find theoretical and observational evidence that this anti-correlation arises from polycyclic aromatic hydrocarbons (PAHs). Growth of PAHs in molecular clouds strengthens the 2175 Angstrom feature and causes R(V) to decrease. R(V) variation from larger grains (silicates) is ruled out due to the mass budget available in gas phase. If proven, this not only serves as evidence of the in-situ growth of PAHs, but changes our view of what drives R(V) variation and opens a new window towards studying PAH evolution and the carbon cycle in the ISM with optical extinction information.