3D ISM structure challenges the Serkowski relation

The Serkowski relation is the cornerstone of studies of starlight polarization as a function of wavelength. Although empirical, its extensive use since its inception to describe polarization induced by interstellar dust has elevated the relation to the status of an indisputable "law", serving as the benchmark for validating interstellar dust grain models. We revisit the effects of the 3D structure of the interstellar medium (ISM) on the wavelength dependence of interstellar polarization. We use analytical models to show how the wavelength dependence of both the polarization fraction and direction is affected by the presence of multiple clouds along the line of sight (LOS), accounting for recent developments in dust distribution modelling and utilizing an expanded archive of stellar polarization measurements. We highlight concrete examples of stars whose polarization profiles are severely affected by LOS variations of the dust grain and magnetic field properties, and we provide a recipe to accurately fit multiple cloud Serkowski models to such cases. We present, for the first time, compelling observational evidence that the 3D structure of the magnetized ISM often results to the violation of the Serkowski relation. We show that 3D effects impact interstellar cloud parameters derived from Serkowski fits. In particular, the dust size distribution in single - cloud sightlines may differ from analyses that ignore 3D effects, with important implications for dust modelling in the Galaxy. Our results suggest that multiband stellar polarization measurements offer an independent probe of the LOS variations of the magnetic field, constituting a valuable new tool for the 3D cartography of the ISM. We caution that, unless 3D effects are explicitly accounted for, a poor fit to the Serkowski relation does not, by itself, constitute conclusive evidence that a star is intrinsically polarized.