Accurate Parallax Measurement toward the Symbiotic Star R Aquarii

Abstract

Multi-epoch phase-referencing VLBI (Very Long Baseline Interferometry) observations with VERA (VLBI Exploration of Radio Astrometry) were performed for the symbiotic star R Aquarii (R Aqr) from September 2005 to Oct 2006. Tracing one of the $v=2$, $J=1-0$ SiO maser spots, we measured an annual parallax of $\pi = 4.59\pm0.24$ mas, corresponding to a distance of $218_{-11}^{+12}$ pc. Our result is consistent with earlier distance measurements, but yields the highest accuracy of about $5\%$ level. Applying our distance, we derived an absolute K-band magnitude of $M_{\mathrm{K}} = -7.71 \pm 0.11$, which is consistent with the recent Period-Luminosity relation by VLBI parallax measurements for 5 OH-Mira variables. In addition, the expansion age of an inner nebulae around R Aqr is found to be about 240 years, corresponds to about the year 1773.