Extreme Starlight Polarization Efficiency toward <i>ζ</i> Ophiuchi: A Case for Line-of-sight Foreground Subtraction

Abstract Despite the pervasive nature of interstellar dust and its impact on nearly all observations, most dust corrections employ line-of-sight averages over large angular scales. This neglects real variations on small angular and distance scales from discrete components of the interstellar medium (ISM). We use V- band polarimetry, public dust maps, and Gaia Data Release 3 distances of 25 stars along a 50′ radius sight line toward the O9.5IV star ζ Ophiuchi ( d ≈ 182 pc) to examine both dust and magnetic structures over the range d = 36–1176 pc and angular scales of &lt;1°. Polarization and reddening data indicate two discrete dust populations having different magnetic field orientations along the sight line, one at d ≃ 86–127 pc and another at d ≃ 252–287 pc. After removal of the foreground, the more distant component exhibits alignment in polarization angle with 12 μ m polycyclic aromatic hydrocarbon (PAH) striations seen in the field. This more distant dust population exhibits evidence of extreme starlight polarization efficiency, with an average of 14.1% mag −1 , greater than the canonical Serkowski limit of 9% mag −1 . The spatial coincidence with the PAH striations indicates the PAH-emitting grains and those responsible for the high polarization efficiency may be components of the same dust population. We find no evidence that ζ Oph’s radiative influence affects the polarizing or reddening properties of the surrounding dust. Our study demonstrates that accurate distance-based foreground subtraction is vital to properly understanding superimposed dust and the magnetic field components in the ISM.