The Age of Globular Clusters in Light of Hipparcos: Resolving the Age Problem?
Abstract
We review five independent techniques that are used to set the distance scale to globular clusters, including subdwarf mainsequence fitting utilizing the recent Hipparcos parallax catalog. These data together all indicate that globular clusters are farther away than previously believed, implying a reduction in age estimates. We now adopt a bestfit value M_{v} (RR Lyrae stars) = 0.39 +/ 0.08 (statistical) at [Fe/H] = 1.9 with an additional uniform systematic uncertainty of ^{+0.13}_{0.18}. This new distance scale estimate is combined with a detailed numerical Monte Carlo study (previously reported by Chaboyer et al.) designed to assess the uncertainty associated with the theoretical ageturnoff luminosity relationship in order to estimate both the absolute age and uncertainty in age of the oldest globular clusters.
Our best estimate for the mean age of the oldest globular clusters is now 11.5 +/ 1.3 Gyr, with a onesided 95% confidence level lower limit of 9.5 Gyr. This represents a systematic shift of over 2 σ compared to our earlier estimate, owing completely to the new distance scalea shift which we emphasize results not only from the Hipparcos data. This now provides a lower limit on the age of the universe that is consistent with either an open universe or with a flat matterdominated universe (the latter requiring H_{0} <= 67 km s^{1} Mpc^{1}). Our new study also explicitly quantifies how remaining uncertainties in the distance scale and stellar evolution models translate into uncertainties in the derived globular cluster ages. Simple formulae are provided that can be used to update our age estimate as improved determinations for various quantities become available. Formulae are also provided that can be used to derive the age and its uncertainty for a globular cluster, given the absolute magnitude of the turnoff or the point on the subgiant branch 0.05 mag redder than the turnoff.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 1998
 DOI:
 10.1086/305201
 arXiv:
 arXiv:astroph/9706128
 Bibcode:
 1998ApJ...494...96C
 Keywords:

 COSMOLOGY: THEORY;
 COSMOLOGY: DISTANCE SCALE;
 GALAXY: GLOBULAR CLUSTERS: GENERAL;
 STARS: EVOLUTION;
 STARS: INTERIORS;
 STARS: POPULATION II;
 Cosmology: Theory;
 Cosmology: Distance Scale;
 Galaxy: Globular Clusters: General;
 Stars: Evolution;
 Stars: Interiors;
 Stars: Population II;
 Astrophysics
 EPrint:
 41 pages, including 10 eps figs, uses aaspp4.sty and flushrt.sty, submitted to Ap.J., revised to incorporate FULL Hipparcos catalogue data