Gold alignment & internal dissipation

Abstract

The measures of mechanical alignment were obtained for both prolate and oblate grains when their temperature is comparable with grain kinetic energy devided by k, the Boltzmann constant. For such grains, the alignment of angular momentum, J, with the axis of maximal inertia, a, is only partial. This substantially alters the alignment as compared with the results in Lazarian (1995) and Roberge, Hanany & Messinger (1996) obtained on the assumption of perfect alignment. We also describe the Gold alignment when the Barnett dissipation is suppressed and derive an analytical expression which relates the measure of alignment with parameters of grain nonsphericity and the direction of the gas - grain drift. This solution provides the lower limit for the alignment measure, while the upper limit is given by the analytics derived in Lazarian (1994). Using results of a recent study of incomplete internal relaxation in Lazarian & Roberge (1996), we find measures of alignment for the whole range of ratios of grain rotational energy to k over T_s, where T_s is the grain temperature. To describe alignment for mildly supersonic drifts, we suggest an analytical approach which provides good correspondence with the results of direct numerical simulations in Roberge, Hanany & Messinger (1995). We also extend our approach to account for the simultaneous action of the Gold and Davis-Greenstein mechanisms.

25 pages, 14 figures. Accepted to ApJ

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