High-Velocity Stars in Globular Clusters

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Session 104 -- Globilar Clusters
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[104.11] High-Velocity Stars in Globular Clusters

Ruth C. Peterson (Lick)

We have surveyed datasets of velocities of stars in globular clusters to see whether any stars are genuinely moving at velocities in excess of the apparent escape velocity defined by stars within a few core radii of the center. For several globulars, datasets are available which contain over 100 stars with published individual velocities and velocity errors, which are complete (no measurements have been summarily discarded), and in which the membership of each individual star may be established unambiguously. In several cases, proper motions are also available and three-dimensional velocities may be calculated. We calculate the velocity dispersion $\sigma$ from the velocities of all stars interior to the radius at which the velocity dispersion is expected to drop by no more than 10\%. From this and the cluster concentration, we estimate a nominal escape velocity, which is typically 4 to 5$\sigma$ from the cluster mean velocity. We then subtract the cluster velocity from the velocity observed for each star, and normalize the result to the local value of $\sigma$ by dividing by the fractional change in the velocity dispersion expected at the star's apparent radial distance for a cluster of the given concentration.

We find no stars whose velocities significantly exceed 5$\sigma$, though almost every cluster has one or more stellar velocities above 3$\sigma$. We thus conclude that the vast majority of such stars are natural outliers of the truncated Maxwellian velocity distribution. This includes in particular the two stars in 47 Tucanae of Meylan, Dubath, \& Mayor (1991, ApJ, 383, 587) and the two stars in M3 of Gunn \& Griffin (1979, AJ, 84, 752); in both cases the authors omitted the outliers in the calculation of the velocity dispersion. We also conclude that globular clusters are unlikely to be imbedded in dark halos with substantial mass but little light. The presence of such a halo would allow stars with higher velocities to remain bound to a globular, yet none are seen.

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