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Session 97 - Galactic Structure, Galactic Center.
Oral session, Friday, January 09

[97.06] The Proper Motion of Sgr A*

M. J. Reid (Harvard-Smithsonian CfA), A. C. S. Readhead, R. Vermeulen (Caltech), R. Treuhaft (JPL)

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.

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