3D interstellar medium structure challenges the Serkowski relation

Context. The Serkowski relation is the cornerstone of studies of starlight polarizations as a function of wavelength. Although empirical, its extensive use since its inception in 1975 to describe polarization induced by interstellar dust has elevated the relation to the status of an indisputable “law”: this is the benchmark against which models of interstellar dust grains are validated. Aims. We revisit the effects of the 3D structure of the interstellar medium (ISM) on the wavelength dependence of interstellar polarization. Methods. 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). We account for recent developments in dust distribution modeling and we utilize an expanded archive of stellar polarization measurements to establish the effect of multiple clouds along the LOS. We highlight concrete examples of stars whose polarization profiles are severely affected by LOS variations in the dust grain and magnetic field properties, and we provide a recipe to accurately fit multiple cloud Serkowski models to such cases. Results. We present, for the first time, compelling observational evidence that the 3D structure of the magnetized ISM often results in 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 modeling in the Galaxy. Conclusions. Our results suggest that multiwavelength stellar polarization measurements offer an independent probe of the LOS variations in the magnetic field orientation, and thus constitute a potentially valuable new tool for the 3D cartography of the ISM. Finally, 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.