Interstellar Polarization in M31^*

The wavelength dependence of interstellar polarization due to dust in M31 has been observed along four sight lines. Only one sight line had been measured previously. The globular clusters, S78, S150, S233, and Baade 327 were used as point sources to probe the interstellar dust in M31. The Serkowski law produces good fits for all the sight lines, although the relationship between K and λ_max may be different from that found in the Galaxy. The results of this study imply that the slope K/λ_max may be significantly larger in M31. The Serkowski curves are significantly narrower than those of the same λ_max in the Galaxy and may require extreme modifications to the size distributions of silicate particles. The fits for the four sight lines reveal values of λ_max ranging from 4800 to 5500 Å. These are consistent with average values of λ_max measured in the Galaxy and the Magellanic Clouds. The range measured for M31 corresponds to R_V values of 2.7–3.1. The range in R_V seen in the Galaxy is 2.5–5.5 implying, for this small sample, that the average size of interstellar grains in M31 is typically smaller than that seen for Galactic grains if the nature of the grains is the same. Also, the polarization efficiency for these sight lines is large, although some bias is expected since sight lines known to have significant interstellar polarization were selected for the sample.