The circumstellar envelopes around OH/IR stars

We have conducted high spatial resolution observations of the 1612-MHz OH maser emission from five OH/IR stars using MERLIN in its spectral-line mode. The observations yield the angular radii of the circumstellar envelopes. These can be combined with the linear radii obtained from phase-lag techniques to calculate source distances. In each case the distances obtained differ significantly from the kinematic estimates and imply peculiar velocities of between 10 and 30 km s−1. We have also conducted single-baseline observations to determine the absolute positions of nine OH/IR objects, and find that they are coincident with the optical positions of the stars, where these positions are available. We have collated all published 1612-MHz OH maser maps of OH/IR stars in order to study the shell structure. We find that the observations are generally consistent with the expanding-shell model, although significant asymmetries are present. The principal asymmetry is a front-back spatial offset between the peaks of maser emission. There is no observable offset at the velocity extremes. We interpret the offset between the emission peaks in terms of velocity perturbations within the envelope and are able to put a firm upper limit of 1.7 km s−1 on the perturbing velocity. The two other asymmetries are (1) the lack of complete rings of emission, which we interpret in terms of density or optical depth variations within the shell, and (2) a difference in shell radius between the blue and redshifted sides for two sources. We suggest that the unusual source W43(OH) is undergoing decelerating bipolar outflow.