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We present new, more precise measurements of the mass and distance of ourGalaxy's central supermassive black hole, Sgr A*. These results stem from a newanalysis that more than doubles the time baseline for astrometry of faint starsorbiting Sgr A*, combining two decades of speckle imaging and adaptive opticsdata. Specifically, we improve our analysis of the speckle images by usinginformation about a star's orbit from the deep adaptive optics data (2005 -2013) to inform the search for the star in the speckle years (1995 - 2005).When this new analysis technique is combined with the first completere-reduction of Keck Galactic Center speckle images using speckle holography,we are able to track the short-period star S0-38 (K-band magnitude = 17,orbital period = 19 years) through the speckle years. We use the kinematicmeasurements from speckle holography and adaptive optics to estimate the orbitsof S0-38 and S0-2 and thereby improve our constraints of the mass ($M_{bh}$)and distance ($R_o$) of Sgr A*: $M_{bh} =4.02\pm0.16\pm0.04\times10^6~M_{\odot}$ and $7.86\pm0.14\pm0.04$ kpc. Theuncertainties in $M_{bh}$ and $R_o$ as determined by the combined orbital fitof S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, comparedto an orbital fit of S0-2 alone and a factor of $\sim$2.5 compared to previousresults from stellar orbits. This analysis also limits the extended dark masswithin 0.01 pc to less than $0.13\times10^{6}~M_{\odot}$ at 99.7% confidence, afactor of 3 lower compared to prior work.

An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis