Thermal Flipping and Thermal Trapping -- New Elements in Dust Grain Dynamics

Abstract

Since the classical work by Purcell (1979) it has been generally accepted that most interstellar grains rotate suprathermally. Suprathermally rotating grains would be nearly perfectly aligned with the magnetic field by paramagnetic dissipation if not for ``crossovers'', intervals of low angular velocity resulting from reversals of the torques responsible for suprathermal rotation; during crossovers grains are susceptible to disalignment by random impulses. Lazarian and Draine (1997) identified thermal fluctuations within grain material as an important component of crossover dynamics. For grains of size less than 0.1 micron, these fluctuations ensure good correlation of angular momentum before and after crossover resulting in good alignment, in accord with observations of starlight polarization. In the present paper we discuss two new processes which are important for the dynamics of grains with a<0.1 micron. The first -- ``thermal flipping'' -- offers a way for small grains to bypass the period of greatly reduced angular momentum which would otherwise take place during a crossover, thereby enhancing the alignment of small grains. The second effect -- ``thermal trapping'' -- arises when thermal flipping becomes rapid enough to prevent the systematic torques from driving the grain to suprathermal rotation. This effect acts to reduce the alignment of small grains. The observed variation of grain alignment with grain size would then result from a combination of the thermal flipping process -- which suppresses suprathermal rotation of small grains -- and due to molecular hydrogen formation and starlight -- which drive large grains to suprathermal rotation rates.