AAS Meeting #193 - Austin, Texas, January 1999
Session 68. Globular Clusters
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[68.02] HST Internal Proper Motions at the Center of the Globular Cluster 47 Tucanae

J. Anderson, I.R. King (UC Berkeley), G. Meylan (ESO)

We have significantly improved the level of accuracy of differential astrometry with the Hubble Space Telescope. Whereas we were previously able to separate globular-cluster stars from field stars, we can now measure the internal dispersion of proper motions in a cluster with high accuracy. Using a 2-year time interval with HST's WFPC2 camera, we have measured internal proper motions for 4000 stars in the globular cluster 47 Tuc, with an error for each star that averages only 1/4 of the rms motion of stars. Comparison of the proper-motion dispersion with that of radial velocities should yield a fundamental distance to the cluster that is considerably more accurate than distances found by other methods.

Our images were taken at the cluster center, through the F300W filter, which suppresses the light of the bright giants and allows faint and bright stars to be measured almost equally well. Using precise centering techniques and locally based coordinate transformations, we measure the proper motion of a typical cluster star relative to its neighbors with a precision of a fraction of a milliarcsec per year. The mean position of a star at one epoch has an error of less than 0.01 of a PC pixel.

The position of a star measured on a single image has an error of about 0.03 of a PC pixel. Achieving the \surd N advantage of averaging N measurements is far from trivial, however, requiring careful attention to systematic errors that may arise from the undersampling at the WFPC2 pixel scales.

Also, calculation of a reliable rms proper motion requires reliable correction for the measuring errors, a step that demands extreme care in the statistical treatment of the data.

The ability to make these measurements opens up a whole new range of possibilities, not only for improving the distance scale, but also for studying the internal kinematics of clusters:\ anisotropies, dependence of velocity dispersion on mass, etc.

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