New space motion of Galactic globular cluster Palomar 5

A new space motion of the Galactic globular cluster Pal 5 has been obtained on the basis of the proper motion determination from 11 Schmidt plates with a time baseline of 39 years. A field of 1 square degree with nearly 400 galaxies defining the absolute reference frame has been used in the differential astrometry from plate-to-plate solutions. Residual systematic errors after the solutions with 2 x 6 plate constant polynomials have been investigated and removed. Typical proper motion errors of 2 to 3 mas/yr were obtained for individual stars with magnitudes 12 < B < 20. Although our result for the mean absolute proper motion of Pal 5 (mu(alpha)/l(delta)) = (-1.0 +/- 0.3, -2.7 +/- 0.4) mas/yr looks similar to the result of Schweitzer, Cudworth & Majewski (1993) based on different observations and reduction technique, the direction of the space motion we derive is completely different from theirs. Although the position of Pal 5 is near the orbital plane suggested for the Sagittarius dwarf galaxy by Ibata et al. (1997), both proper motion results suggest a total space velocity of Pal 5 significantly different from that to be expected from a tidally stripped fragment of the Sgr dwarf galaxy. In addition, our direction of motion of Pal 5 is almost in the opposite sense to that of the Sgr galaxy. Our Galactic orbit determination of Pal 5 and the star counts on the deep Schmidt plates indicate a smaller tidal radius of this globular cluster compared to previously published data of Webbink (1985) and Trager, King & Djorgovski (1995).