Hubble Space TelescopeObservations of the Post–Core‐Collapse Globular Cluster NGC 6752. II. A Large Main‐Sequence Binary Population

We present a color-magnitude diagram (CMD) of NGC 6752 based on post-refurbishment Planetary Camera 2 observations of its core. The main sequence is broadened and asymmetric, as would be expected if there were large numbers of binary stars. We use artificial star experiments to characterize the broadening of the main sequence that is expected, due to both photometric errors and the effect of chance superposition of stars. The observed broadening is significantly greater than can be explained by these two effects alone, so a main-sequence binary population is required to explain the observations. We develop a Monte Carlo technique to calculate the binary frequency in the CMD. The binary fraction is probably in the range 15%-38% in the inner core radius (r < 11'') but is probably less than 16% beyond that.