**Previous
abstract** **Next
abstract**

**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**