An Update on Monitoring Stellar Orbits in the Galactic Center
  Using 25 years of data from uninterrupted monitoring of stellar orbits in the
Galactic Center, we present an update of the main results from this unique data
set: A measurement of mass of and distance to SgrA*. Our progress is not only
due to the eight year increase in time base, but also due to the improved
definition of the coordinate system. The star S2 continues to yield the best
constraints on the mass of and distance to SgrA*; the statistical errors of
0.13 x 10^6 M_sun and 0.12 kpc have halved compared to the previous study. The
S2 orbit fit is robust and does not need any prior information. Using
coordinate system priors, also the star S1 yields tight constraints on mass and
distance. For a combined orbit fit, we use 17 stars, which yields our current
best estimates for mass and distance: M = 4.28 +/- 0.10|stat. +/. 0.21|sys. x
10^6 M_sun and R_0 = 8.32 +/- 0.07|stat. +/- 0.14|sys. kpc. These numbers are
in agreement with the recent determination of R_0 from the statistical cluster
parallax. The positions of the mass, of the near-infrared flares from SgrA* and
of the radio source SgrA* agree to within 1mas. In total, we have determined
orbits for 40 stars so far, a sample which consists of 32 stars with randomly
oriented orbits and a thermal eccentricity distribution, plus eight stars for
which we can explicitly show that they are members of the clockwise disk of
young stars, and which have lower eccentricity orbits.
