THE UNUSUAL OH ENVELOPE OF U ORIONIS

MERLIN maps of the OH maser emission from U Orionis reveal an incomplete ring of 1665-MHz masers of radius 1015 em and two compact clusters of 1612-MHz masers. The 1612-MHz masers are separated along the same position angle at which the break occurs in the ring of 1665-MHz masers. The ring distribution of 1665-MHz masers can be explained in terms of a thin-shell model for the masers. The model implies a stellar velocity of -39.5±1.0kms-1. The model also makes a number of predictions which it may be possible to test as the maser emission from U Ori continues to evolve. The 1665-MHz masers show a regular velocity-gradient around the ring. No simple explanation for this is completely satisfactory, although it seems that rotation of the circumstellar envelope probably contributes to the gradient observed. The stellar magnetic field may provide the necessary coupling to transfer angular momentum from the star to the circumstellar gas. © Royal Astronomical Society • Provided by the NASA Astrophysics Data System.