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We report new precision measurements of the properties of our Galaxy'ssupermassive black hole. Based on astrometric (1995-2007) and radial velocity(2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fullyunconstrained Keplerian orbit for the short period star S0-2 provides valuesfor Ro of 8.0+-0.6 kpc, M_bh of 4.1+-0.6x10^6 Mo, and the black hole's radialvelocity, which is consistent with zero with 30 km/s uncertainty. If the blackhole is assumed to be at rest with respect to the Galaxy, we can furtherconstrain the fit and obtain Ro = 8.4+-0.4 kpc and M_bh = 4.5+-0.4x10^6 Mo.More complex models constrain the extended dark mass distribution to be lessthan 3-4x10^5 Mo within 0.01 pc, ~100x higher than predictions from stellar andstellar remnant models. For all models, we identify transient astrometricshifts from source confusion and the assumptions regarding the black hole'sradial motion as previously unrecognized limitations on orbital accuracy andthe usefulness of fainter stars. Future astrometric and RV observations willremedy these effects. Our estimates of Ro and the Galaxy's local rotationspeed, which it is derived from combining Ro with the apparent proper motion ofSgr A*, (theta0 = 229+-18 km/s), are compatible with measurements made usingother methods. The increased black hole mass found in this study, compared tothat determined using projected mass estimators, implies a longer period forthe innermost stable orbit, longer resonant relaxation timescales for stars inthe vicinity of the black hole and a better agreement with the M_bh-sigmarelation.

Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits