Abstract
Radial velocities have been measured for nine blue horizontal-branch stars in the globular cluster M 4. Their mean velocity is 72.3 ± 0.9 km/s, and their velocity dispersion (after removal of the ±1.5 km/s random error on an individual measurement) is 1.9 ± 0.3 km/s. The observed dispersion is considerably smaller than the value of 4.4 km.s inferred by Peterson and King (1975) from their fit of a King model to the central surface brightness distribution, assuming a solar mass-to-light ratio. This new measurement is more discrepant with such a fit than that noted by Petrson and King, who relied upon the central velocity dispersions measured in integrated light by Illingworth (1976). Of necessity, the clusters studied by Illingworth are highly centrally concentrated, while M 4 is of rather low central concentration. This new result raises the possibility that M 4 in particular, and low-concentration clusters in general, may have mass-to-light ratios that are less than solar. This suggestion should be checked using accurate measurements (to better than 1 km/s) of the velocities of a dozen giants in this and other low-concentration clusters.
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