Simulations of polarized dust emission

Aims. The aim is to study the polarization of thermal dust emission to see if the alignment of the grain by radiative torques could explain the observed relation between the degree of polarization and intensity. Predictions are made for polarimetry observations with the Planck satellite. Methods. Our results are based on model clouds derived from MHD simulations of magnetized turbulent flows. The continuum radiative transfer problem is solved with Monte Carlo methods to estimate the three-dimensional distribution of dust emission and the radiation field strength affecting the grain alignment. The minimum grain size aligned by the radiative torques is calculated, and the Rayleigh polarization reduction factor, R , is derived for different grain size distributions. We show maps of polarized thermal dust emission that are predicted by the models. The relation between the intensity and polarization degree is examined in self-gravitating cores. Furthermore, we study the effects of wavelength, resolution, and observational noise. Results. We are able to reproduce the P/I-relation with the grain alignment by radiative torques. The decrease in intrinsic polarization and total emission means that sub-mm polarimetry carries only a little information about the magnetic fields in dense cores with high visual extinction. The interpretation of the observations will be further complicated by the unknown magnetic field geometry and the fact that what is observed as cores may, in fact, be a superposition of several density enhancements. According to our calculations, Planck will be able to map dust polarization reliably when A V exceeds ∼ 2 m at spatial resolution of ~$ 15'$.