Polarization and the Ratio of Total-to-Selective Extinction

Relationships between extinction and polarization parameters are explored using ultraviolet (ANS and IUE), visual, and infrared data along the same lines of sight. In particular, the well-known empirical relationship between polarization and extinction, p ≤ 9.0E(B-V), has been re-examined using lines of sight for which R_V, the ratio of total-to-selective extinction, is known. It is found that the amount of polarization per unit extinction p/A_V is related to R_V such that the maximum observed value of p/A_V is smaller for large R_V. This implies that large grains are not efficient at producing polarization, which may be attributed to a reduced axial ratio or to less efficient alignment.