| We study the color-magnitude diagram 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 if a main-sequence binary population is required to account for the observed asymmetric broadening. We find that the most probable binary fraction fb in the center of M3 lies between 6% and 22%. For those stars with distances from the cluster center of M3 between one and two core radii, the most probable value of fb lies between 1% and 3%. The similar radial distributions of binaries and blue stragglers 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 MS stars of about 0.013 to 0.014 magnitude 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 MS stars in M13.;With the observations of globular cluster o Cen through the Advanced Camera for Surveys (ACS) on Hubble Space Telescope, the radial distributions of blue stragglers (BSs) and blue subdwarfs (sdBs) in the cluster are studied. No significant differences between radial distributions of BSs and the corresponding reference populations are found, which implies that no signature of mass segregation is evident in o Cen and the cluster is dynamically unrelaxed. Collisional origin should not be responsible for the majority of the BSs observed in o Cen. Radial distributions of bright and faint sdBs are studied, the hints for different models of sdBs origin are discussed. |
