The Galactic Center: An Improved Astrometric Reference Frame for Stellar
  Orbits around the Supermassive Black Hole
  Precision measurements of the stars in short-period orbits around the
supermassive black hole at the Galactic Center are now being used to constrain
general relativistic effects, such as the gravitational redshift and periapse
precession. One of the largest systematic uncertainties in the measured orbits
has been errors in the astrometric reference frame, which is derived from seven
infrared-bright stars associated with SiO masers that have extremely accurate
radio positions, measured in the Sgr A*-rest frame. We have improved the
astrometric reference frame within 14'' of the Galactic Center by a factor of
2.5 in position and a factor of 5 in proper motion. In the new reference frame,
Sgr A* is localized to within a position of 0.645 mas and proper motion of 0.03
mas/yr. We have removed a substantial rotation (2.25 degrees per decade), that
was present in the previous less-accurate reference frame used to measure
stellar orbits in the field. With our improved methods and continued monitoring
of the masers, we predict that orbital precession predicted by General
Relativity will become detectable in the next ~5 years.
