AAS Meeting #193 - Austin, Texas, January 1999
Session 65. Interstellar Dust and Gas
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[65.01] Thermal Flipping - A New Element of Dust Grain Dynamics

A. Lazarian (Canadian Institute for Theoretical Astrophysics), B.T. Draine (Princeton University Observatory)

Since the classical work by Purcell (1979) it has been generally accepted that grains rotate suprathermally, i.e., with angular velocities much greater than thermal. Several independent mechanisms to drive such rotation have been identified, seemingly ensuring substantial suprathermal rotation for all except the very smallest grains. 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 the low angular momentum makes the grain susceptible to disalignment by random impulses.

Lazarian and Draine (1997) identified thermal fluctuations within grain material as an important component of crossover dynamics. They found that these fluctuations ensure good correlation of angular momentum before and after crossover resulting in good alignment of grains larger than 10-5 cm. This conclusion is in accord with observations showing that starlight polarization is produced by a >10-5 cm grains. We show that a new feature of thermal fluctuations -- which we refer to as ``thermal flipping'' -- implies that the degree of alignment should be high for small grains as well, if small grains rotate suprathermally. However, observations indicate that grains with a <10-5 cm are not appreciably aligned. This implies that a< 10-5 cm grains do not rotate suprathermally, contrary to expectation.

We argue that the thermal flipping phenomenon itself can suppress suprathermal rotation of a<10-5 cm 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 radiative torques due to starlight -- which drive large grains to suprathermal rotation rates.

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