Alignment of suprathermally rotating grains

Abstract

It is shown, that mechanical alignment can be efficient for suprathermally rotating grains, provided that they drift with supersonic velocities. Such a drift should be widely spread due to both Alfv\'{e}nic waves and ambipolar diffusion. Moreover, if suprathermal rotation is caused by grain interaction with a radiative flux, it is shown, that mechanical alignment may be present even in the absence of supersonic drift. This means that the range of applicability of mechanical alignment is wider that it is generally accepted and it can rival the paramagnetic one. We also study the latter mechanism and reexamine the interplay between poisoning of active sites and desorption of molecules blocking the access to the active sites of H$_{2}$ formation in order to explain the observed poor alignment of small grains and good alignment of large grains. To have a more comprehensive picture of alignment we briefly discuss the alignment by radiation fluxes and caused by grain magnetic moments.