2018ApJ...869...80A

We imaged silicon monoxide masers toward the asymptotic giant branch star R Cas at 23 intervals covering almost 2 pulsation cycles. The masers are concentrated in a shell within 4 stellar radii. Between 19 and 62 features were identified per epoch and 184 of these were matched at 3 or more epochs, forming 38 series. The features probably survive more epochs than their detectable masers. We compared the proper motions and polarization of these clumps, providing the first complete assessment of the net expansion velocity over more than one cycle and the significance of the magnetic field. Proper motions are irregular, dominated by outflow (infall) in the first (second) cycles. Sixty-five matched pairs had maser polarization angles consistent within π/8. A small excess (22) of this subsample has proper motion vectors within ±22.°5 of being parallel to the inferred magnetic field, 10 of which have approximately radial proper motions. The average field strength needed to provide a magnetic energy density equivalent to the bulk kinetic and thermal energy densities of a clump is ∼2 G, similar to direct magnetic field measurements. While some clumps possess a consistent magnetic field, capable of influencing the direction of motion, only a minority flow along magnetic field lines. The resultant SiO expansion proper motion over the entire periods is ∼0.4 km s^–1 (taking ∼67 yr to cross the shell), which, compared with the mass in the shell, implies a wind similar to mass-loss rates from the literature measured on larger scales.