On the shape of interstellar grains

The scattering properties of homogeneous dielectric spheroids of arbitrary size are calculated using a recent analytical solution given by Asano and Yamamoto. These calculations are used to investigate the extinction and polarization produced by spheroidal interstellar grain models. Comparison is made with three other methods of dealing with nonspherical grains: the theory of anomalous diffraction, microwave experiments, and calculations for infinite cylinders. We discuss the effects of grain shape on both the observed laws of extinction (through the value of R) and polarization. Although these effects are difficult to isolate, the maximum value of the ratio of polarization to extinction in the V passband can be used with a theory of grain alignment to place useful restrictions on the shape and refractive index of homogeneous spheroidal grains.