A Tale of Three Dust Populations: Variable $R_{\rm{V}}$ and Extreme Polarization Along Sightlines Toward $\zeta$ Ophiuchi

TL;DR

This study characterizes three distinct dust populations along sightlines toward $z Ophiuchi, revealing variations in dust properties, polarization efficiency, and grain size distribution in different interstellar environments.

Contribution

It identifies and characterizes three separate dust components with distinct properties and locations, providing new insights into dust variation in a highly irradiated diffuse cloud environment.

Findings

Three dust populations with different $R_V$ and polarization properties identified.

Highly polarizing mid-distance dust associated with $z Ophiuchi.

Distant dust component contains larger grains, above the Galactic Plane.

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.