VERA Observations of SiO Masers in the Symbiotic Star R Aquarii

We present phase-referenced maps of the SiO maser transitions v = 1 J = 1–0 and v = 2 J = 1–0 from the symbiotic stellar system R Aquarii, which hosts an evolved AGB star plus a hot companion. Observations were performed at two epochs: 2004.98 and 2005.98. Accurate absolute coordinates and proper motions of the emission centroid were obtained; the errors expected for these parameters are also given. We compare the VERA data with the previous astrometry by Hipparcos. This represents a possibility to improve the orbital parameters of the system in a different way than that done before. Thanks to our accurate astrometry, we have also estimated the percentage of spot coincidences between both maser transitions, a parameter that has been proposed to be relevant to discriminate between different maser pumping schemes. Although the overall distributions of both lines are always similar, the spots are rarely coincident, in a percentage ranging between 3% and 20% of the cases. The lack of systematic coincidence favors, in principle, radiative pumping. However, we argue that no firm conclusion can be reached due to a lack of models that include an overlap of rovibrational lines, and that accurately address the coincidence of very intense spots of different maser transitions.