Dust beyond R(V)

Gregory Green

Tuesday, Dec. 3rd, 16:00CET

The optical-NIR dust extinction curve is typically parameterized by a single variable, R(V), which controls the slope of the curve. R(V) is thought to reflect the grain-size distribution and composition of dust. Low-resolution, flux-calibrated BP/RP spectra from Gaia have allowed the determination of the extinction curve along sightlines to 130 million stars in the Milky Way and Magellanic Clouds. We show that these extinction curves contain more than a single degree of freedom - that is, that they are not simply described by R(V). We identify at least three physically meaningful components that are orthogonal to R(V) variation, and show that these components vary across the sky in coherent patterns that resemble interstellar medium structure. These components encode variation in the 770 nm extinction feature, intermediate-scale and very broad structure, and a newly identified feature at 850 nm, and likely trace both dust composition and local conditions in the interstellar medium. Correlations between R(V) and the 770 and 850 nm features suggest that the carriers of these features become more abundant as the carrier of the 2175 Angstrom feature is destroyed. The availability of densely sampled maps of extinction-curve structure beyond R(V) opens up the possibility of more detailed analyses of the rich chemistry of the interstellar medium.