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 t o obtain high angular resolution JHK images of the centers of the metal-poo r globular clusters NGC 5272 (M3), NGC 6205 (M13), NGC 6287, and NGC 6341 ( M92). The color-magnitude diagrams (CMDs) derived from these data include t he upper main sequence and most of the red giant branch and agree with publ ished photometric measurements of bright stars in these clusters. The photo metric accuracy is limited by point-spread function variations, which intro duce systematic errors of a few hundredths of a magnitude near the referenc e star. The NGC 6287 CMDs are of particular interest, as they are free of t he scatter caused by foreground stars and differential reddening that have plagued previous efforts to study the stellar content of this inner spheroi d 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 m orphology versus metallicity relation defined by other clusters in the inne r Galaxy. The clusters are paired according to metallicity, and the near-infrared CMD s 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 para meter. We tentatively conclude that NGC 6287 and NGC 6341 have ages that di ffer by no more than +/-2 Gyr. However, the near-infrared spectral energy d istributions of stars in NGC 6287 appear to differ from those of stars in o uter halo clusters, bringing into question the validity of this age estimat e.