Stellar OH masers and magnetic fields - VLBI observations of U Orionis and IRC +10420

Abstract

This letter reports single-antenna and VLBI interferometric observations of the unusual stellar hydroxyl masers U Ori and IRC +10420. It is found that the apparent sizes of all the maser components from -50 to -40 km/s with flux densities greater than 1 Jy in U Ori have increased by at least an order of magnitude over a three-year period, indicating a change in the physical conditions of the masing region, that the integrated 1612-MHz flux density of U Ori decreased by a factor of four over the same period, and that the emission is partly circularly polarized but less than 5% linearly polarized. An interferometric position for the OH maser in IRC +10420 is determined and shown to agree with the optical position within the errors. Magnetic-field strengths of about 10 mG are estimated for the masing regions of both sources, along with surface magnetic fields of approximately 10 G for U Ori and 100 G for IRC +10420.