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Session 97 - Galactic Structure, Galactic Center.
Oral session, Friday, January 09
Georgetown,
Since 1991, we have conducted observations with the VLBA designed to measure the distance to the Galactic Center via a trigonometric parallax. A time series of measurements of the position of Sgr A* relative to extragalactic sources should show the effects of the annual \approx\pm0.12 mas signature of the Earth's orbit around the Sun (trigonometric parallax), the \approx6 mas yr^-1 secular motion caused by the Sun's orbit around the Galactic Center, and perhaps the motion of Sgr A* itself. \vskip 0.2 truecm We have demonstrated an accuracy of about 0.1 and 0.4 mas in the East-West and North-South directions, respectively, for the position of Sgr A* relative to background quasars. This has allowed us to measure the angular rotation rate of the Sun around the Galactic Center (\Theta_0/R_0)with an uncertainty of about 8% over a time span of only 1 year. Assuming a flat rotation curve for the Galaxy, this also provides a direct measurement of Oort's constants with a similar uncertainty. \vskip 0.2 truecm Finally, our observations can put stringent limits, or perhaps detect, any peculiar motion of Sgr A* (beyond that expected from our moving vantage point at the Sun). If Sgr A* is a very massive (e.g., >10^6 M_ødot) black hole at the dynamical center of the Galaxy, one would expect a peculiar motion of order 1 km s^-1, owing to gravitational perturbations from close encounters with massive stars in the central star cluster. However, were Sgr A* to be a lower mass system, commensurate with its modest total luminosity, it would be expected to have a much larger peculiar motion.
