R Aquarii: Evidence for Differential Rotation of the S[CLC]i[/CLC]O Maser Shell

Abstract

We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v=1, J=1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v~(1/rq)1/2. We find that q~1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian.