Radiative torques of irregular grains: Describing the alignment of a
  grain ensemble
  The radiative torque (RAT) mechanism is the most promising way of explaining
observed polarization arising from aligned grains. We explore the efficiency of
the grain alignment by an anisotropic radiation flow for an extensive ensemble
of grain shapes. We calculate the distribution of the ratios of the amplitudes
of the two major components of the RATs, that is an essential parameter that
enters the theory of RAT alignment in Lazarian & Hoang (2007, LH07). While this
distribution is different for different classes of grain shapes that we
considered, the most probable values of the parameter are centered in the range
of $q^{max}\sim 0.5-1.5$. The functional form from RATs calculated is in good
agreement with the analytical model (AMO). We find that the RAT efficiency
scales as $(\lambda/a)^{-3}$ for $\lambda\gg a$ as previously found in LH07.
This increases the power of predictions obtained with the RAT theory. We also
confirm that superparamagnetic inclusions are necessary in achieving high
degrees of alignment, and constrain the parameter space describing the
requirements for achieving these alignment degrees.
