Observational Study of the Galactic Nuclear Disk using SiO Masers

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In a three-year research during 2001-2003, we performed the following observations. 1. A shallow survey of SiO maser sources in a area of 20 arcminutes toward the galactic center using a multi-beam receiver. 2. A survey of300 Large Amplitude Variables in SiO masers in the Galactic nuclear disk. 3. VLBA observations of Sgr A^* and SiO maser sources around it. S. Deguchi mainly concerned with observations 1 and 2, and associated data reductions, and M. Miyoshi concerned with the observation 3. With observations 1 and 2, we found that there are several stars with very high radial velocities, and that the rotational velocity field consists of two components. With monitoring observations of the Galactic center SiO masers, we found that the masers associated with IRs10 flared again during May - December 2002. Therefore, we made VLBI observations again in 2003 January. The data are now under processing by M. Miyoshi. From the observations made in July 2000, we obtained the proper motion of IRs10 relative to Sgr A^*. In addition, we performed the high resolution imaging of Sgr A^*. According to this analysis, it was tuend out that the intensity and emission distribution of Sgr A^* varies within a few hours. Because this is due to flares of Sgr A^*, we discovered the variation of the Sgr A^* flare first in the world. These new results were presented in the international symposium at Hawaii. We still have several Large Amplitude Variables unobserved and will finish observations this summer.