A New Self-consistency Check on the Ages of Globular Clusters

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Abstract
We present a new method to compute stellar ages in globular clusters (GCs) that is 10 times more precise than the traditional isochrone-fitting procedure. The method relies on accurate stellar evolutionary tracks and on photometry for GCs complete down to the main sequence and is based on counting the numbers of stars in two different regions of the color-magnitude diagram: the red giant branch and the main sequence. We have applied this method to the globular cluster M68 and found an age of 16.4 ± 0.2 Gyr for (m - M)V = 15.3. This new method reduces the error associated with the uncertainty in the distance modulus by a factor of 2, the error due to the choice of the value for the mixing-length parameter to almost zero, and the error due to the color-Teff transformation to zero.