Predictions of polarized dust emission from interstellar clouds: spatial
  variations in the efficiency of radiative torque alignment
  Polarization carries information about the magnetic fields in interstellar
clouds. The observations of polarized dust emission are used to study the role
of magnetic fields in the evolution of molecular clouds and the initial phases
of star-formation. We study the grain alignment with realistic simulations,
assuming the radiative torques to be the main mechanism that spins the grains
up. The aim is to study the efficiency of the grain alignment as a function of
cloud position and to study the observable consequences of these spatial
variations. Our results are based on the analysis of model clouds derived from
MHD simulations. The continuum radiative transfer problem is solved with Monte
Carlo methods to estimate the 3D distribution of dust emission and the
radiation field strength affecting the grain alignment. We also examine the
effect of grain growth in cores. We are able to reproduce the results of Cho &
Lazarian using their assumptions. However, the anisotropy factor even in the 1D
case is lower than their assumption of $\gamma = 0.7$, and thus we get less
efficient radiative torques. Compared with our previous paper, the polarization
degree vs. intensity relation is steeper because of less efficient grain
alignment within dense cores. Without grain growth, the magnetic field of the
cores is poorly recovered above a few $A_{\rm V}$. If grain size is doubled in
the cores, the polarization of dust emission can trace the magnetic field lines
possibly up to $A_{\rm V} \sim 10$ magnitudes. However, many of the prestellar
cores may be too young for grain coagulation to play a major role. The
inclusion of direction dependent radiative torque efficiency weakens the
alignment. Even with doubled grain size, we would not expect to probe the
magnetic field past a few magnitudes in $A_{\rm V}$.
