THE SHAPES AND ALIGNMENT PROPERTIES OF INTERSTELLAR DUST GRAINS

We use the results of far-infrared polarimetry to set limits on the shapes and degrees of alignment of dust grains in interstellar clouds. Assuming spheroidal grains the upper limit to the ratio of the short axis to the long axis is similar to 0.9 if the grains are oblate and similar to 0.8 if they are prolate. By comparing the results of midinfrared spectropolarimetry with calculated polarization spectra we confirm that the grains are oblate and estimate that the ratio of axes is not lower than similar to 1/3. The best fit is obtained for a ratio of similar to 2/3. Assuming a ratio similar to 2/3 (oblate) we infer that the maximum degrees of polarization observed in the far-infrared are within a factor of similar to 4 of the value for perfect alignment and approximately equal to the value for optimum alignment by ambipolar diffusion.