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The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers.However, previous VLBI observations have been performed separately, which makesit difficult to spatially trace the outward transfer of the materialconsecutively. Here we present the astrometrically registered, simultaneousmaps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VXSagittarii. The H2O masers detected above the dust-forming layers have anasymmetric distribution. The multi-transition SiO masers are nearly circularring, suggesting spherically symmetric wind within a few stellar radii. Theseresults provide the clear evidence that the asymmetry in the outflow isenhanced after the smaller molecular gas clump transform into the inhomogeneousdust layers. The 129.3 GHz maser arises from the outermost region compared tothat of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser ismaximized around the optical maximum, suggesting that radiative pumping isdominant.

Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii