KaVA ESTEMA Project

Hiroshi Imai, Se-Hyung Cho, Yoshiharu Asaki, Yoon-Kyong Choi, Jaeheon Kim, Youngjoo Yun, Naoko Matsumoto, Cheul-Hong Min, Tomoaki Oyama, S.-C. Yoon, D.-H. Kim, Richard Dodson, Maria J. Rioja, Ross, A. Burns, Gabor Orosz , Miyako Oyadomari, Akiharu Nakagawa, James Chibueze O., Juni-ichi Nakashima & Andrey M. Sobolev

Kagoshima University, Japan

The KaVA (The combined array of the Korean VLBI Network and Japanese VLBI Exploration of Radio Astrometry) ESTEMA (Expanded Study on Stellar Masers) Project is one of three planed Large Programs launched in this year. ESTEMA aims to publish a database of the largest sample of VLBI images of circumstellar water (H2O) and silicon-monoxide (SiO) maser sources towards 80 evolved stars in late AGB to early post-AGB phase. Therefore, it enables statistical analyses of the masers found among different types of the evolved stars at a variety of stellar pulsation phases, from microscopic (individual maser spots) to macroscopic (circumstellar envelope, CSE) views. In ESTEMA, for each star the relative locations and distributions of different maser lines are directly compared. Thus it enables us to analyze the dependence of maser pumping mechanisms on stellar type and pulsation phase, plus the evolution of asymmetric stellar mass loss found through biased H2O maser spatio-kinematic structures in the CSE with respect to the SiO maser location, pinpointing the central star. These sources of information will lead us to understanding the basic properties of circumstellar H2O and SiO masers as probes of the physical conditions and dynamics of CSEs. The ESTEMA outputs will become a legacy reference for planning any long-term intensive monitoring campaigns of evolved stars exhibiting high mass loss rates, with current large and forthcoming radio astronomy facilities including KaVA and ALMA.

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