Accurate Parallax Measurement toward the Symbiotic Star R Aquarii

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 π=4.59±0.24 mas, corresponding to a distance of 218+12−11 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 MK=−7.71±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.

Authors:  Cheulhong Min, Naoko Matsumoto, Mi Kyoung Kim, Tomoya Hirota, Katsunori M. Shibata, Se-Hyung Cho, Makoto Shizugami and Mareki Honma

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