Strömgren Photometry of Globular Clusters: The Distance and Age of M13, Evidence for Two Populations of Horizontal-Branch Stars

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Strömgren uvbyβ system and determine a foreground reddening of E(b-y)=0.015±0.01 mag. From a fit to the main sequence of metal-poor subdwarfs with Hipparcos parallaxes, we derive (m-M)0=14.38±0.10, which implies an age near 12 Gyr assuming [Fe/H] = -1.61 and [α/Fe] = 0.3. The distance-independent [(b-y)0, c0] diagram indicates that M13 and metal-poor field subdwarfs of similar metallicity must be coeval to within ±1 Gyr. In addition, we find that, at any given (b-y)0 color, there is a large spread in the c0 index for M13 red giant branch (RGB) stars. We suspect that this scatter, which extends at least as faint as the base of the RGB, is most likely due to star-to-star variations in the atmospheric abundances of the CNO elements. We also note the existence of what appears to be two separate stellar populations on the horizontal branch of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants.