Accurate parallax measurement toward the symbiotic star R Aquarii

Multiepoch 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 2005 September to 2006 October. Tracing one of the v = 2, J = 1-0 SiO maser spots, we measured an annual parallax of π = 4.59 ± 0.24 mas, corresponding to a distance of 218_{-11}^{+12}pc. Our result is consistent with earlier distance measurements, but yields the highest accuracy of ∼ 5% level. Applying our distance, we derived an absolute K-band magnitude of M_K = -7.71 ± 0.11, which is consistent with the recent period-luminosity relation by VLBI parallax measurements for five OH-Mira variables. In addition, the expansion age of an inner nebula around R Aqr is found to be ∼ 240 yr, corresponding to about the year 1773.