2005AJ....129.1934Z

We study the color-magnitude diagram (CMD) morphologies of M3 and M13 using high-precision Hubble Space Telescope photometry. Asymmetric broadening above and to the red of the main sequence is apparent. We use extensive artificial star experiments to synthetically reproduce chance superposition and photometric error and examine whether a main-sequence binary population is required to account for the observed asymmetric broadening. We find that the most probable binary fraction f_b in the center of M3 lies between 6% and 22%. For those stars of M3 with distances from the cluster center between 1 and 2 core radii, the most probable value of f_b_ lies between 1% and 3%. The similar radial distributions of binaries and blue stragglers (BSs) support the model that the BSs in the center of M3 are of collisional origin. A similar comparison between the color distributions of real stars and artificial stars in the CMD of M13 suggests that the real star distributions are consistently broader than the artificial ones. After corrections of real and artificial differences in color uncertainties, we find evidence for an intrinsic color dispersion among main-sequence stars of about 0.013-0.014 mag in both the center and outer regions of M13. This is larger than the expected value resulting from the spectroscopically observed [Fe/H] dispersion and might reflect differences in [C/Fe] and [N/Fe] that have been noted spectroscopically among upper main-sequence stars in M13.