The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*

We report measurements with the VLBA of the position of Sgr A* with respect to two extragalactic radio sources over a period of eight years. The apparent proper motion of \sgrab relative to J1745-283 is 6.379 +/- 0.024 mas/y along a position angle of 209.60 +/- 0.18 degrees, almost entirely in the plane of the Galaxy. The effects of the orbit of the Sun around the Galactic center can account for this motion, and the residual proper motion of Sgr A* perpendicular to the plane of the Galaxy is -0.4 +/- 0.9 km/s. A maximum-likelihood analysis of the motion expected for a massive object within the observed Galactic center stellar cluster indicates that Sgr A* contains more than about 10% of the approximately 4 x…