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Session 71 - Globular Clusters.
Display session, Wednesday, January 15
We have obtained velocities for 230 stars in the globular cluster M15 using the Hydra multi-object spectrograph on the WIYN telescope. This sample of stars extends outwards to 16.6 arc minutes from the cluster center. For comparison, the tidal radius of the cluster has been estimated to lie in the range 20-40' in previous studies. Candidates for spectroscopy were selected from Curtis Schmidt CCD BV photometry and from Cudworth's proper motion list. We selected stars that lie on the giant branch with V>16.6; this faint-end limit is 0.8 mag below the horizontal branch. The stars were observed at Mg b with a spectral resolution of about 20,000. Spectra were obtained for a total of 615 stars. Velocities were determined by cross correlation of the spectra against that of a a multiply observed cluster giant. Typical velocity uncertainties are less than 1\mboxkm\,s^-1 , for the combined observations of each star. Likely cluster members were identified based on velocity consistency with the cluster mean and weak line strength. Of the likely cluster members, we have filtered out nine stars which show evidence of velocity variability. We find that the velocity dispersion decreases outwards to a distance of 5' from the cluster center. However, beyond this radius, the velocity dispersion increases outwards. This behavior has been predicted for a cluster interacting with the tidal field of the Galaxy. The energy input from this interaction acts to heat the outer regions of the cluster, and ultimately causes halo stars to escape. These data provide new information on the outer dynamics of M15 and will constrain new anisotropic Fokker-Planck models that we are developing.
Program listing for Wednesday