A Tale of Three Dust Populations: Variable $R_{\rm{V}}$ and Extreme Polarization Along Sightlines Toward $\zeta$ 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 towards 27 stars surrounding the mid-latitude $b$=$+$24$^{\circ}$ O9.2IV star $\zeta$ Ophiuchi, using new optical spectroscopy and polarimetry. We incrementally deredden and depolarize with distance, allowing us to distinguish dust components along these sightlines. The data indicate three distinct dust populations: a foreground component characteristic of average Milky Way dust ($R_{\rm{V}}$$\approx$3.1, $d$$\lesssim$180 pc), a highly polarizing mid-distance component in the vicinity of $\zeta$ Oph ($R_{\rm{V}}$$\approx$2.4, 200 pc$<$$d$$<$300 pc), and a non-polarizing distant component ($R_{\rm{V}}$$\approx$3.6, 600 pc$<$$d$$<$2000 pc). Prominent 8 $\mu$m infrared striations spanning the field of view likely have high Polycyclic Aromatic Hydrocarbon content and are illuminated by $\zeta$ Oph. Foreground-subtracted polarizations roughly align with these striations, which, we argue, lie immediately behind $\zeta$ Oph and constitute the highly-polarizing mid-distance dust. This component polarizes very efficiently ($P_{\rm{V}}$$>$9.1$E(B-V)$), implying a high degree of grain alignment and suggesting that the bulk of the polarization occurs in a small fraction of the volume. The large $R_{\rm{V}}$ in the distant component reveals that dust above the Galactic Plane ($z$$>$250 pc) may contain a greater fraction of large grains than the Milky Way average.