Which Grains Align? The Radiative-Torque Alignment Size and the Aligned Mass Fraction in Five Environments

Only grains above a certain size are expected to align with the magnetic field by radiative torques, and that size decides how much of the dust mass can polarise light. This paper evaluates an alignment-size scaling with gas density and radiation strength for five environments and converts the result into the aligned mass fraction of a power-law size distribution. In diffuse gas the alignment size is 0.055 microns and the aligned mass fraction is 0.6184; in a dense core the alignment size rises to 1.078 microns and the aligned fraction falls to 0.0; near a massive star strong radiation lowers the alignment size to 0.0402 microns and the aligned fraction rises to 0.6978. In the dense core the alignment size exceeds the largest grains the standard distribution contains, so within that distribution no mass remains aligned at all; polarisation from dense cores is therefore intrinsically suppressed by the loss of aligned mass, quite apart from field geometry, and any polarisation that is observed from such cores must be carried by grains grown beyond the standard distribution, which is itself evidence for that growth. All values are computed from stated scalings, not measured.