Disorientation of interstellar grains in suprathermal rotation

As pointed out by Purcell, interstellar grains are subject to an unbalanced torque which produces spin-up to high angular speeds; magnetic relaxation then aligns the grain rotation with B during the time t/sub r/. However, changes in the grain surface are likely to change these systematic torques, occasionally reversing them; the rotational velocity then decreases to a minimum at ''crossover'' and then increases again. The mean interval between crossovers is uncertain but may be much less than the value of t/sub r/ resulting from paramagnetic relaxation; if so, and if the grains are disoriented during crossover, paramagnetic relaxation then falls short by about an order of magnitude in accounting for the polarization of starlight.This paper computes the cumulative deflection angle, chi/sub T/, of the grain's angular momentum, J, during crossover, using both a simple theoretical model and detailed dynamical computer calculations. The results indicate that the disorientation is relatively complete in two to three crossovers. Hence paramagnetic relaxation theory can readily account for grain orientation only if ''long-lived spin up'' is assumed, with infrequent crossovers.