Extreme Starlight Polarization Efficiency Toward $\zeta$ 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 ISM. We use $V$ band polarimetry, public dust maps, and Gaia DR3 distances of 25 stars along a 50' radius sight line towards the O9.5IV star $\zeta$ Ophiuchi ($d\approx$ 182 pc) to examine both dust and magnetic structures over the range $d=$ 36--1176 pc and angular scales of $<1$\degr. Polarization and reddening data indicate two discrete dust populations having different magnetic field orientations along the sight line, one at $d\simeq$ 86--127 pc and another at $d\simeq$ 252--287 pc. After removal of the foreground, the more distant component exhibits alignment in polarization angle with 12 $\mu$m 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 $\zeta$ 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.

arXiv comment19 pages, 8 figures, Accepted to AJ

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