FABRY-PEROT MEASUREMENTS OF THE DYNAMICS OF GLOBULAR-CLUSTER CORES - M15 (NGC-7078)

We report the first use of the Rutgers Imaging Fabry-Perot Spectrophotometer to study the dynamics of the cores of globular clusters. We have obtained velocities for cluster stars by tuning the Fabry-Perot to take a series of narrowband (0.8 angstrom FWHM at 5890 angstrom) images at different wavelengths across one of the Na D (5890 angstrom) absorption lines. Measuring the flux in every frame yields a short portion of the spectrum for each star simultaneously. This proves to be a very efficient method for obtaining accurate stellar velocities; in crowded regions we are able to measure hundreds of velocities in 3-4 h of observing time. We have measured velocities with uncertainties of less than 5 km s-1 for 216 stars within 1.5' of the center of the globular cluster M15 (NGC 7078). The velocity dispersion profile shows a sharp rise from 7 to 12 km s-1 at 0.6' (1.8 parsec), and then appears to flatten into our innermost point at 0.1'. A rotation amplitude of 1.4 +/- 0.8 km s-1 is detected. The rotation has been measured at a radius of 0.6' using stellar velocities and at 0.2' using the integrated light profile. The amplitude and position angle are the same at both radii, indicating a constant rotation profile in this region. Combining our two epochs of Fabry-Perot observation with published measurements, we have repeat velocity measurements for 67 stars. We calculate a binary fraction of about 7% for binary periods between 0.2 and 20 years and mass ratios larger than 0.22, which is in agreement with measurements for other globular clusters.