1 0 A ug 2 00 4 Grain Alignment in Molecular Clouds

Polarimetry is one of the most informative techniques of studying magnetic fields in molecular clouds. How reliable the interpretation of the polarization maps in terms of magnetic fields is the issue that the grain alignment theory addresses. We show that grain alignment involves several processes acting simultaneously, but on different timescales. We explain that rotating dust grains get substantial magnetic moment that allows them precess fast about magnetic field lines. As the result, grains preserve their orientation to magnetic field when the magnetic field direction fluctuates. We point out to the importance of internal alignment, i.e. the process forces grain axes to be aligned in respect to the grain angular momentum. We show that subtle quantum effects, in particular relaxation related to nuclear magnetic moments of atoms composing the grain, brings to live complex grain motions, e.g. flips. These flips substantially alter the dynamics of grain and limit the applicability of earlier theories that did not account for them. We also briefly review basic physical processes involved in the alignment of grain angular momentum in respect to interstellar magnetic field. We claim that the bulk of existing observational data is consistent with the radiative torque alignment mechanism. In particular, we show that large grains that are known to exist in the cores of molecular clouds may be aligned by the attenuated external interstellar radiation field. 1. Why do we care? The fact that interstellar grains get aligned has been puzzling researchers for more than half a century. Very soon after the discovery of grain alignment by Hall (1949) and Hiltner (1949) it became clear that the alignment happens in respect to magnetic field. Since that time grain alignment stopped to be the issue of pure scientific curiosity, but became an important link connecting polarimetry observations with the all-important interstellar magnetic fields 1. The history of grain alignment ideas is exciting (see review by Lazarian 2003) but we do not have space here to dwell upon it. Last decade has been marked by a substantial progress in understanding new physics associated with grain alignment. The theory has become predictive, which enables researchers to interpret observational data with more confidence. Within this short review we discuss the modern understanding of grain alignment processes applicable to molecular clouds. We discuss both internal alignment, i.e. the alignment of grain axes in respect to grain angular momen-1 Additional interest to grain alignment arises from recent …