A cosmic battery

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents that give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it also seems very promising as a way to account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.