High angular resolution JHK imaging of the centers of the metal-poor globular clusters NGC 5272 (M3), NGC 6205 (M13), NGC 6287, and NGC 6341 (M92)

The Canada-France-Hawaii Telescope Adaptive Optics Bonnette has been used to obtain high angular resolution JHK images of the centers of the metal-poor globular clusters NGC 5272 (M3), NGC 6205 (M13), NGC 6287, and NGC 6341 (M92). The color-magnitude diagrams (CMDs) derived from these data include the upper main sequence and most of the red giant branch and agree with published photometric measurements of bright stars in these clusters. The photometric accuracy is limited by point-spread function variations, which introduce systematic errors of a few hundredths of a magnitude near the reference star. The NGC 6287 CMDs are of particular interest, as they are free of the scatter caused by foreground stars and differential reddening that have plagued previous efforts to study the stellar content of this inner spheroid cluster. The horizontal branch (HB) of NGC 6287 contains stars spanning a broader range of temperatures than that of NGC 6341. NGC 6287 is thus the only known member of the metal-poor inner spheroid to deviate from the HE morphology versus metallicity relation defined by other clusters in the inner Galaxy. The clusters are paired according to metallicity, and the near-infrared CMDs and luminosity functions are used to investigate the relative ages within each pair. The near-infrared CMDs provide the tightest constraints on the relative ages of the classical second-parameter pair NGC 5272 and NGC 6205 and indicate that these clusters have ages that differ by no more than +/-1 Gyr. These results thus support the notion that age is not the second parameter. We tentatively conclude that NGC 6287 and NGC 6341 have ages that differ by no more than +/-2 Gyr. However, the near-infrared spectral energy distributions of stars in NGC 6287 appear to differ from those of stars in outer halo clusters, bringing into question the validity of this age estimate.