A Tale of Three Dust Populations: Variable R _V and Extreme Polarization along Sight Lines toward ζ Ophiuchi

Abstract Dust permeates the interstellar medium, reddening and polarizing background starlight, but dust properties vary with local environment. In order to characterize dust in a highly irradiated diffuse cloud, we measure the reddening and optical polarization toward 27 stars surrounding the midlatitude b = +24° O9.2IV star ζ Ophiuchi, using new optical spectroscopy and polarimetry. We incrementally deredden and depolarize with distance, allowing us to distinguish dust components along these sight lines. The data indicate three distinct dust populations: a foreground component characteristic of average Milky Way dust (R V ≈ 3.1, d ≲ 180 pc), a highly polarizing mid-distance component in the vicinity of ζ Oph (R V ≈ 2.4, 200 pc < d < 300 pc), and a nonpolarizing distant component (R V ≈ 3.6, 600 pc < d < 2000 pc). Prominent 8 μm infrared striations spanning the field of view likely have high polycyclic aromatic hydrocarbon content and are illuminated by ζ Oph. Foreground-subtracted polarizations roughly align with these striations, which, we argue, lie immediately behind ζ Oph and constitute the highly polarizing mid-distance dust. This component polarizes very efficiently (P V > 9.1E(B − V)), implying a high degree of grain alignment and suggesting that the bulk of the polarization occurs in a small fraction of the volume. The large R V in the distant component reveals that dust above the Galactic plane (z > 250 pc) may contain a greater fraction of large grains than the Milky Way average.