c 2008. Astronomical Society of Japan. Relative Astrometry of the J = 1!0, v = 1 and v = 2 SiO Masers

Oxygen-rich Asymptotic Giant Branch (AGB) stars are intense emitters of SiO and H2O maser lines at 43 (J = 1!0, v = 1 and 2) and 22 GHz, respectively. VLBI observations of the maser emission provide a unique tool to sample the innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned v = 1 and v = 2 SiO maser maps have traditionally prevented a unique interpretation of the observations in terms of the physical underlying conditions, which depend on the nature of the SiO pumping mechanism. We have carried out observations of the SiO and H2O maser emission towards R LMi, using the astro-metric capabilities of VERA. Due to the too weak emission of the reference calibrator, we had to develop a special method to accurately relate the coordinates for both transitions. We present relative astrometrically aligned v = 1 and v = 2 J = 1!0 SiO maser maps, at multiple epochs, and discuss the astrophysical results. The incorporation of astrometric information into the maps of SiO masers challenges the weak points in the current theoretical models, which will need further refinements to address the observation results