The Position of Sagittarius A*. III. Motion of the Stellar Cusp

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Abstract
In the first two papers of this series, we determined the position of Sgr A* on infrared images by aligning the positions of red giant stars with positions measured at radio wavelengths for their circumstellar SiO masers. In this paper, we report the detections of five new stellar SiO masers within 50'' (2 pc) of Sgr A* and new and/or improved positions and proper motions of 15 stellar SiO masers. The current accuracies are ≈1 mas in position and ≈0.3 mas yr-1 in proper motion. We find that the proper motion of the central stellar cluster with respect to Sgr A* is less than 45 km s-1. One star, IRS 9, has a three-dimensional speed of ≈370 km s -1 at a projected distance of 0.33 pc from Sgr A*. If IRS 9 is bound to the inner parsec, this requires an enclosed mass that exceeds current estimates of the sum of the mass of Sgr A* and luminous stars in the stellar cusp by ≈0.8 × 106 M☉. Possible explanations include: (1) that IRS 9 is not bound to the central parsec and has "fallen" from a radius greater than 9 pc, (2) that a cluster of dark stellar remnants accounts for some of the excess mass, and/or (3) that R0 is considerably greater than 8 kpc.