Thermal Flipping and Thermal Trapping: New Elements in Grain Dynamics

Since the classical work by Purcell (1979) it has been generally acceptedthat most interstellar grains rotate suprathermally. Suprathermally rotatinggrains would be nearly perfectly aligned with the magnetic field byparamagnetic dissipation if not for ``crossovers'', intervals of low angularvelocity resulting from reversals of the torques responsible for suprathermalrotation; during crossovers grains are susceptible to disalignment by randomimpulses. Lazarian and Draine (1997) identified thermal fluctuations within grainmaterial as an important component of crossover dynamics. For grains of sizeless than 0.1 micron, these fluctuations ensure good correlation of angularmomentum before and after crossover resulting in good alignment, in accord withobservations of starlight polarization. In the present paper we discuss two newprocesses which are important for the dynamics of grains with a<0.1 micron. Thefirst -- ``thermal flipping'' -- offers a way for small grains to bypass theperiod of greatly reduced angular momentum which would otherwise take placeduring a crossover, thereby enhancing the alignment of small grains. The secondeffect -- ``thermal trapping'' -- arises when thermal flipping becomes rapidenough to prevent the systematic torques from driving the grain to suprathermalrotation. This effect acts to reduce the alignment of small grains. The observed variation of grain alignment with grain size would then resultfrom a combination of the thermal flipping process -- which suppressessuprathermal rotation of small grains -- and due to molecular hydrogenformation and starlight -- which drive large grains to suprathermal rotationrates.Comment: 16 pages, 2 figures, submitted ApJ