Kinematic Study of the Disrupting Globular Cluster Palomar 5 Using VLT Spectra

Wide-field photometric data from the Sloan Digital Sky Survey have recentlyrevealed that the Galactic globular cluster Palomar 5 is in the process ofbeing tidally disrupted (Odenkirchen et al. 2001). Here we investigate thekinematics of this sparse remote star cluster using high resolution spectrafrom the Very Large Telescope (VLT). Twenty candidate cluster giants locatedwithin 6 arcmin of the cluster center have been observed with the UV-VisualEchelle Spectrograph (UVES) on VLT-UT2. The spectra provide radial velocitieswith a typical accuracy of 0.15 km/s. We find that the sample contains 17certain cluster members with very coherent kinematics, two unrelated fielddwarfs, and one giant with a deviant velocity, which is most likely a clusterbinary showing fast orbital motion. From the confirmed members we determine theheliocentric velocity of the cluster as -58.7 +- 0.2 km/s. The totalline-of-sight velocity dispersion of the cluster stars is 1.1 +- 0.2 km/s (allmembers) or 0.9 +- 0.2 km/s (stars on the red giant branch only). This is thelowest velocity dispersion that has so far been measured for a stellar systemclassified as a globular cluster. The shape of the velocity distributionsuggests that there is a significant contribution from orbital motions ofbinaries and that the dynamical part of the velocity dispersion is thereforestill substantially smaller than the total dispersion. ... (abridged)Comment: 29 pages including 10 figures, accepted for publication in the Astronomical Journa