NONACCELERATION OF SAGITTARIUS-A-ASTERISK - IMPLICATIONS FOR GALACTICSTRUCTURE

We estimate that observations by Backer and collaborators over the pas t two decades constrain the time derivative of the proper motion of Sg r A to be less than 0.14 mas yr(-2). Using this result and a measurem ent by Eckart & Genzel of sigma similar to 560 km s(-1) for the veloci ty dispersion of the star cluster within 0.'' 13 of Sgr A, we derive the following implications. First, if the nuclear star cluster is domi nated by a massive black hole, then either Sgr AY: is directly associa ted with that black hole or it orbits the black hole with a radius les s than or similar to 40 AU. Under this assumption, and adopting upsilo n(LSR) = 220 +/- 10 km s(-1), the Galacto-centric distance is therefor e R-0 = 7.5 +/- 0.4 kpc. If the star cluster is not assumed to contain a massive black hole, Sgr A is constrained to move slower than 55 km s(-1) (1 sigma) relative to the center of mass of the cluster. This i s too weak a limit to provide useful constraints on Galactic structure , but we show that it could be improved by more than one order of magn itude over the next decade. Such an improvement would eliminate the ne ed to make any assumption about a black hole in the derivation of R-0.