Thermal flipping and thermal trapping: New elements in grain dynamics

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. Lazarian & Draine (1997) identified thermal fluctuations within grain material as an important component of crossover dynamics. For a ≳ 10^-5 cm grains, these fluctuations ensure good correlation of angular momentum before and after crossover resulting in good alignment, in accord with observations of starlight polarization. We discuss two new processes that are important for the dynamics of a ≲ 10^-5 cm grains. "Thermal flipping" allows small grains to bypass the period of slow rotation that would otherwise occur during a crossover, thereby enhancing the alignment of small grains. "Thermal trapping" arises when thermal flipping becomes rapid enough to prevent the systematic torques from driving the grain to suprathermal rotation, thereby reducing the alignment of small grains. The observed variation of grain alignment with grain size results from a competition between thermal trapping - which suppresses suprathermal rotation of small grains - and torques due to H₂ formation and starlight - which drive large grains to suprathermal rotation rates.