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

Details Stromgren Photometry of Globular Clusters: The Distance and Age of M13, Evidence For Two Populations of Horizontal-Branch Stars Stromgren Photometry of Globular Clusters: The Distance and Age of M13, Evidence For Two Populations of Horizontal-Branch Stars

1998-06-04

arxiv.org/abs/astro-ph/9806081v1

Astronomy and Astrophysics

Astrophysics

14 pages, including two encapsulated postscript figures. Accepted for publication in ApJL, Uses aasms4.sty

Scientific paper

10.1086/311419

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Stromgren uvby-Beta system, and determine a foreground reddening of E(b-y)= 0.015mag. 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 [Alpha/Fe]=0.3. The distance independent ((b-y)_0, c_0) 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 c_0 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 HB 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.

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