THE SHAPES AND ALIGNMENT PROPERTIES OF INTERSTELLAR DUST GRAINS

We use the results of far-infrared polarimetry to set limits on the sh apes and degrees of alignment of dust grains in interstellar clouds. A ssuming spheroidal grains the upper limit to the ratio of the short ax is to the long axis is similar to 0.9 if the grains are oblate and sim ilar to 0.8 if they are prolate. By comparing the results of midinfrar ed 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 sim ilar to 2/3. Assuming a ratio similar to 2/3 (oblate) we infer that th e maximum degrees of polarization observed in the far-infrared are wit hin a factor of similar to 4 of the value for perfect alignment and ap proximately equal to the value for optimum alignment by ambipolar diff usion.