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P.R. Wozniak (LANL), S. Kozlowski (Univ. of Manchester), W.T. Vestrand (LANL), S. Mao (Univ. of Manchester)
The observed optical depths to microlensing of stars in the Galactic Bulge are difficult to reconcile with our present understanding of Galactic dynamics. The main source of uncertainty in those comparisons is now shifting from microlensing measurements to the dynamical models of the Galactic bar. Velocity measurements in the Bulge provide sensitive constraints on possible scenarios. We present a kinematic study of the Bulge stars in 35 fields centered on sources that were previously detected as microlensing events by the MACHO experiment. Proper motions in the reference frame of a mean star along the line of sight are derived from a comparison between the archival WFPC/PC images and our second epoch imaging using ACS/HRC detector. Target fields cover an area about 3 deg across in the vicinity of Baade's window (l=2.5, b=-3) deg. With typical time baselines ~4-7 years we can reliably measure spatial gradients of the apparent velocity dispersion. Our preliminary measurements indicate that the velocity gradient along Galactic latitude b is weaker than predicted by published models of the Galactic bar. An average lensed star in the Bulge is expected to be farther than a randomly selected star. Therefore, the mean motion of the lensed sources relative to all stars in the line of sight carries information on kinematics as a function of distance and the importance of streaming motions.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.