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DEEP IR IMAGING OF GLOBULAR CLUSTERS. III. M13

We have used the CFHT REDEYE infrared camera to obtain deep J and K' images of four fields in the globular cluster M13. The composite K, J-K color-magnitude diagram (CMD) extends from the upper red giant branch (RGB) to 2 magnitudes below the main sequence turn-off (MSTO). Selected [Fe/H]~ -1.6 isochrones from Bergbusch & VandenBerg (1992, ApJS, 81, 163) and Straniero & Chieffi (1991, ApJS, 76, 525) are transformed onto the near-infrared observational plane, and these suggest an age for M13 in the range 14-16 Gyr. We emphasize that any effort to estimate ages should be considered as tentative given uncertainties in the input physics; however, these uncertainties notwithstanding, comparisons between the near-infrared isochrones indicate that age differences of order +/-2 Gyr should be detectable among metal-poor clusters on the K, J-K plane. Building on this result, we find that the difference in J-K color between the MSTO and the base of the RGB in M13 is the same as in M4, and conclude that these clusters have similar ages. This conclusion is verified by comparing (1) the K brightnesses of the MSTO, and (2) the morphologies of optical CMDs. Finally, we investigate the mass function of main sequence stars in M13 with distances between 3 and 6 core radii from the cluster center. The mass functions in the interval 0.55 and 0.8 solar masses are relatively flat, independent of radius. Optical studies at larger radii have found non-zero mass function indices in this same mass interval, and we attribute this radial variation in mass function morphology to dynamical evolution.