2010MNRAS.406.2218E

We present the first view of the magnetic field structure in the OH shell of the extreme OH/infrared star OH26.5+0.6. Multi-Element Radio-Linked Interferometer Network (MERLIN) interferometric observations of this object were obtained in 1993 December in full polarization, at 1612, 1665 and 1667MHz. The maser spots show a spheroidal distribution at both 1612 and 1667MHz, while at 1665MHz emission from the blueshifted maser peak is concentrated on the stellar position and the redshifted peak emission exhibits a filamentary structure oriented on a south-east-north-west axis. The linear polarization in both main lines is rather faint, ranging from 9 to 20 per cent at 1665MHz and from 0 to 30 per cent at 1667MHz. At 1612MHz, most maser spots exhibit a similar range of linear polarization although those in the outermost parts of the envelope reach values as high as 66 per cent. This is particularly apparent in the southern part of the shell. The detailed distribution of the polarization vectors could only be obtained at 1612MHz. The polarization vectors show a highly structured distribution indicative of a poloidal magnetic field inclined by 40°-60° to the line of sight. The velocity distribution of the maser spots with respect to the radial distance is well explained by an isotropic outflow at constant velocity in the case of a prolate-shaped spheroid envelope, also tilted about 45°-65° to the line of sight.