A study on evolved stars by simultaneous observations of H2O and SiO masers using KVN

The Korean VLBI Network (KVN) is a unique millimeter VLBI system which is consisted of three 21 m telescopes with relatively short baselines. We present the preliminary results of simultaneous monitoring observations of the 22.2 GHz H₂O and 43.1/42.8/86.2/129.3 GHz SiO masers based on the KVN Key Science Project (KSP). We obtained the astrometrically registered maps of the H₂O and SiO masers toward nine evolved stars using the source frequency phase referencing method (SFPR). The SFPR maps of the H₂O and SiO masers enabled us to investigate the spatial structure and kinematics from the SiO to H₂O maser regions including the development of an outward motion from the ring-like or elliptical structures of SiO masers to the asymmetric structures of the 22.2 GHz H₂O maser features. In particular, the 86.2/129.3 GHz SiO (v=1, J=2-1 and J=3-2) masers were clearly imaged toward several objects for the first time. The SiO v=1, J=3-2 maser shows different distributions compared to those of the SiO v=1, 2, J=1-0 and v=1, J=2-1 masers implying a different physical condition.