Paramagnetic grain alignment associated with internal friction and the polarization efficiency

Alignment mechanisms associated with paramagnetic relaxation are re-investigated for spheroidal dielectric grains. By taking into account the tight coupling of grain axis with angular momentum due to effective dissipation of rotational energy by internal friction, the degree of alignment by paramagnetic torque will be distinctly improved as compared with that by the classical Davis-Greenstein process. Incorporated with new results by Rogers and Martin on the polarizing effectiveness of spheroidal grains, the ''improved'' DG theory is used to calculate the polarization efficiency due to incompletely aligned grains. It is found that oblate grains with axial ratio of 1/2 or less and size of 0.1 μm can explain the observed polarization if the magnetic field strength is ≅ 3 μG in the general interstellar medium where nH ≅ 1 cm-3 and Tg ≅ 100 K. Alignments of suprathermally spinning grains are also investigated for various grain models. The alignment efficiency is found to depend strongly on the frequency of crossover and the grain shape. (author)