R Aquarii: Constraints on the Rotational Period of the Long-Period Variable

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Abstract
We report Very Large Array (VLA) observations taken in 1996 November and 1998 May of the v = 1, J = 1-0, SiO maser line and BIMA array observations taken in 1999 December and 2000 February of the v = 1, J = 2-1, SiO maser line associated with the long-period variable (LPV) in the R Aquarii binary system that suggest rotation of the maser shell. From these interferometric data cubes, we determine that the maser shell rotation axis is approximately northeast-southwest, thus aligning approximately with the direction of the R Aqr jet; the sense of the maser shell rotation is such that northwest is approaching and southeast is receding; the period of rotation is ~17 yr. Alternatively, co-adding 72 time series spectra of the v = 1, J = 1-0, SiO maser line obtained during the period 1984 July-1990 May with a single-dish antenna, we constructed a composite spectral emission envelope that shows the LSR velocity limits of maser emission over this epoch. From this composite spectral emission envelope and Very Long Baseline Array observations in 1996 February of the v = 1, J = 1-0, SiO maser line, which show the maximal spatial extent of the maser shell, we obtain a shell rotation period of ~18 yr, which is in excellent agreement with the VLA and BIMA array results and represents the maximum rotation period of the LPV if corotating with the maser shell. On the other hand, we obtain a minimum rotation period for the LPV of ~5 yr if the LPV supplies material to the maser shell under the constraint of conservation of angular momentum. The ~5-18 yr range for the rotational period of the LPV determined here and the ~18 yr rotational period for the hot companion determined by previous investigators suggest that tidal effects at successive periastron passages in the R Aqr binary system are tending to synchronize these stellar rotational periods to the orbital period of ~44 yr.

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