Interstellar Polarization in M31

The wavelength dependence of interstellar polarization due to dust in M31 hasbeen observed along four sightlines. Only one sightline had been measuredpreviously.The globular clusters, S78, S150, S233 and Baade 327 were used aspoint sources to probe the interstellar dust in M31. The Serkowski law producesgood fits for all the sightlines although the relationship between K andlambda(max) may be different from that found in the Galaxy. The results of thisstudy imply that the slope K/lambda(max) may be significantly larger in M31.The Serkowski curves are significantly narrower than those of the samelambda(max) in the Galaxy and may require extreme modifications to the sizedistributions of silicate particles. The fits for the four sightlines revealvalues of lambda(max) ranging from 4800 to 5500 A. These are consistent withaverage values of lambda(max) measured in the Galaxy and the Magellanic Clouds.The range measured for M31 corresponds to R(V) values of 2.7 to 3.1. The rangein R(V) seen in the Galaxy is 2.5 to 5.5 implying, for this small sample, thatthe average size ofinterstellar grains in M31 is typically smaller than thatseen for Galactic grains if the nature of the grains is the same. Also, thepolarization efficiency for these sightlines is large although some bias isexpected since sightlines known to have significant interstellar polarizationwere selected for the sample.Comment: 13 pages, 2 figures, AJ in press (June