Systematic variations in the wavelength dependence of interstellar linear polarization

This paper presents new observations of the wavelength dependence of interstellar linear polarization [p(λ)] made to investigate the influence of the environment on the effective size distribution of the aligned polarizing particles. Optical and infrared measurements were obtained simultaneously in eight photometric passbands between U and K, giving a coherent data set for a total of 105 reddened stars. The p(λ) data were modeled using the usual Serkowski relation with three parameters: Ρ_max, λ_max, and K. The added flexibility provided by the third parameter K offers a real improvement in fitting Ρ(λ) in the wavelength range 0.36-2.0 μm. Further modifications to the functional form of Ρ(λ) are necessary as the wavelength range is extended further. We confirm that variations of K and λ_max are correlated, and have revised slightly the empirical linear relationship between K and λ_max found by Wilking et al. to K = 0.01 ± 0.05 + (1.66 ± 0.09)λ_max. There is some cosmic scatter of the data about this line. A new finding is that the same linear dependence of K on λ_max seen overall provides a consistent representation of the systematic polarization changes within individual regions with rather differing environments. Our qualitative explanation is that the grain size distribution in dense regions is modified by coagulation which removes the smaller particles without major modification of the larger ones. Alternatively, the size distribution in diffuse clouds is altered with respect to dense regions by the appearance of smaller particles, without major modification of the larger ones.