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Session 7 - Cosmology and Cosmological Parameters.
Display session, Monday, January 15
North Banquet Hall, Convention Center

[7.02] Uncertainties in the Absolute Age of the Globular Clusters

B. Chaboyer (CITA), P. J. Kernan, L. M. Krauss (CWRU), P. Demarque (Yale)

As the longstanding discrepancy between the expansion age of the universe and the age of the oldest globular clusters continues, it becomes critically important to provide a realistic estimate of the error associated with the globular cluster age estimates. We report the results of a detailed numerical study designed to answer this question. Utilizing estimates of the uncertainty range (and distribution) in the input parameters of stellar evolution codes we produced 1000 Monte Carlo realizations of stellar isochrones, with which we could determine the ages of the 18 oldest globular clusters using the difference in magnitude between the main-sequence turnoff and the horizontal branch. Incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones, we derived a probability distribution for the mean age of these systems. Our best estimate is 14.6\pm 1.7 Gyr, with the one-sided 95% C.L. lower bound of 12.1 Gyr. The total error budget is dominated by the uncertainty in the absolute magnitude of the horizontal branch. Simply varying this quantity over its full 2\sigma range, keeping all other parameters fixed, would produce a \pm 16% change in globular cluster ages estimates. Other significant input parameter uncertainties in this same sense are [\alpha/Fe] (\pm 7 % effect), mixing length (\pm 5% effect), and diffusion, ^14Np reaction rate, the choice of colour table, and the primordial Helium abundance, each of which would affect age estimates at the \pm3% level if allowed to vary over its entire range, keeping all other parameters fixed. Simple formulae are provided which can be used to update our age estimate and its error as improved determinations for the above quantities become available.

Program listing for Monday