INTERSTELLAR POLARIZATION AND MAGNETIC ALIGNMENT OF PINWHEEL DUST GRAINS .1. SUPRATHERMAL GRAIN ROTATION

With the aim of understanding magnetic alignment of non-spherical dust grains inside dense interstellar clouds, the present paper outlines a new model for the Purcell pinwheel mechanism. We assume that polarizing dust grains are oblate spheroids having a classical core-mantle structure. The core consists of small refractory solids (silicates or mixed oxides), whereas the mantle material is composed of a dirty ice mixture. We discuss a model in which cold (T-0 approximate to 10 K) mantles are covered with H-2 molecules accreted from the ambient cloud. When galactic subcosmic rays strike the mantle, they create temporary hotspots on the surface. The hydrogen molecules physisorbed within a hotspot boundary are then ejected from the mantle surface like rockets. In this model, suprathermal rotation of pinwheel grains is still driven by random impulse torques produced by H-2 molecule ejections. We find that pinwheel rotation rates strongly depend on the mantle composition and the chemical nature of the dense cloud interior. Inside reducing clouds, icy mantles are H2O- or CO2- dominated, and pinwheels can rotate suprathermally (omega approximate to 10(8) rad s(-1)), even if gas and dust have a common temperature. If CO condenses on to H2O ice mantles inside oxidizing clouds, however, then icy grains cannot rotate suprathermally. We show that these conditions are consistent with infrared spectroscopic observations of icy grain mantles in dark clouds. We also discuss the survival of pinwheel grains in diffuse interstellar clouds. We conclude that interstellar shocks can destroy the grains; and that the polarization of starlight cannot be understood unless galactic dust is continuously cycled between diffuse and dense cloud phases.