Interferometric observations of four protostellar OH maser sources

Four OH maser sources, NGC 7538, W51, W75S and W75N, have been observed with an interferometer of 24-km baseline. The relative positions of the spectral features in the 1665-MHz spectrum have been measured in each case and for NGC 7538 the 1667-MHz features have also been mapped. In addition, new determinations of the absolute positions of the first three sources have been made. From the resulting maps, estimates of the magnetic flux density and other physical parameters are deduced. The maps show several different configurations of masers, and this diversity is interpreted in terms of the evolution of a protostar. One source, W51, shows evidence of two parallel accretion discs, separated by 0.3 pc. (author)