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		| Paper: | 
		VLBI Studies of Circumstellar Masers | 
	 
	
		| Volume: | 
		340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA | 
	 
	
		| Page: | 
		373 | 
	 
	
		| Authors: | 
		Boboltz, D.A. | 
	 
	
	
		| Abstract: | 
		Asymptotic giant branch (AGB) stars represent an important stage of stellar evolution. Astronomical masers are prevalent in the circumstellar envelopes of oxygen-rich AGB stars. Maser emission from the three most common masing molecules, OH, H2O and SiO, traces regions of the circumstellar envelope(CSE) on angular scales from tens of milliarcseconds to a few arcseconds. Masers provide a unique probe of structure, kinematics, and polarization properties in the environments of these stars. In this review I discuss previous VLBI investigations of the three common masing molecules toward AGB stars concentrating on more recent observations. I will also discuss future trends in astronomical maser observations including the move toward higher observing frequencies, the use of circumstellar masers for high-precision astrometry, and coordinated observations with Optical/IR long-baseline interferometers. | 
	 
	
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