Authors | Michael Odenkirchen (^{1}), Eva K. Grebel (^{1}), Walter Dehnen (^{1}), Hans-Walter Rix (^{1}), Kyle M. Cudworth (^{2}) |
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Affiliation | (^{1}) Max-Planck-Institut fuer Astronomie, Heidelberg (^{2}) Yerkes Observatory, University of Chicago |
Submitted to | Astronomical Journal |
Contact | odenkirchen@mpia-hd.mpg.de |
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From the confirmed members we determine the heliocentric velocity of the cluster as -58.7 ± 0.2 km/s. The total (!) line-of-sight velocity dispersion of the cluster stars is 1.1 ± 0.2 km/s (all members) or 0.9 ± 0.2 km/s (stars on the red giant branch only). This is the lowest velocity dispersion that has so far been measured for a stellar system classified as a globular cluster.
The shape of the velocity distribution suggests that there is a significant contribution from orbital motions of binaries and that the dynamical part of the velocity dispersion is therefore still substantially smaller than the total dispersion. Comparing the observations to the results of Monte Carlo simulations of binaries we find that the frequency of binaries in Pal 5 is most likely between 24% and 63% and that the dynamical line-of-sight velocity dispersion of the cluster must be smaller than 0.7 km/s (90% confidence upper limit). The most probable values of the dynamical dispersion lie in the range 0.12 < sigma/(km/s) < 0.41 (68% confidence). Pal 5 thus turns out to be a dynamically very cold system. Our results are compatible with an equilibrium system. We find that the luminosity of the cluster implies a total mass of only 4.5 x 10^3 to 6.0 x 10^3 solar masses. We further show that a dynamical line-of-sight velocity dispersion of 0.32 km/s to 0.37 km/s admits a King model that fits the observed surface density profile of Pal 5 ( with W_{0} = 2.9 and r_{t} = 16' ) and its mass.