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[76.02] Main-Sequence Binary Stars in the Core of NGC 6397

A.S. Bolton, A.M. Cool (San Francisco State U.), J. Anderson (U.C. Berkeley)

Using HST WFPC2 data, we isolate main-sequence binary candidates in the central region of the globular cluster NGC 6397 based on their locations in an I vs.\ V - I color--magnitude diagram. We have largely eliminated field stars from the sample beforehand based on proper motions determined from two sets of position data separated by approximately three years. Binary candidates are fit to models based on the empirically derived main-sequence ridge line for the cluster, and component masses are determined using theoretical mass--luminosity relations appropriate to the cluster. Preliminary results suggest an upper limit of ~ 3% on the binary fraction for stars in the apparent magnitude range 17.0 \leq V₅₅₅ \leq 20.7 (~ 0.8 to 0.5 M_\odot) and binary mass ratios greater than approximately 0.45. We also present preliminary results for the distribution of binaries as a function of primary mass and mass ratio, as well as a comparison of these results to previously published findings for field stars.