Observation of a ring structure in SiO maser emission from late-type stars

We have used the NRAO Very Long Baseline Array (VLBA) to make the first full high-resolution synthesis images of 43 GHz SiO masers around late-type stars. Our observations of the stars TX Cam and U Her reveal well-defined rings of maser emission at ≈2-4 stellar radii. These images are in contrast to earlier, less accurate, fringe-frequency maps of SiO masers which suggested chaotic structures in similar stars. We can place the SiO masers in the so-called extended atmosphere of the star, where the dynamics are very complex and dust grain formation occurs. Our observations demonstrate that the SiO maser outflow is ordered, not random, and that the masers are being tangentially amplified. We are unable to distinguish between proposed models for radiative and collisional pumping of the SiO masers.