The age of globular clusters in light of {ital Hipparcos}: resolving the age problem?
- Steward Observatory, University of Arizona, Tucson, Arizona85721 (United States)
- Department of Astronomy, Case Western Reserve University, 10900 Euclid Ave., Cleveland, Ohio 44106-7079 (United States)
We review five independent techniques that are used to set the distance scale to globular clusters, including subdwarf main-sequence fitting utilizing the recent {ital Hipparcos} parallax catalog. These data {ital together} all indicate that globular clusters are farther away than previously believed, implying a reduction in age estimates. We now adopt a best-fit value M{sub v} (RR Lyrae stars) =0.39{plus_minus}0.08 (statistical) at [Fe/H]={minus}1.9 with an additional uniform systematic uncertainty of {sub {minus}0.18}{sup +0.13}. This new distance scale estimate is combined with a detailed numerical Monte Carlo study (previously reported by Chaboyer {ital et al.}) designed to assess the uncertainty associated with the theoretical age-turnoff 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{plus_minus}1.3Gyr, with a one-sided 95{percent} confidence level lower limit of 9.5 Gyr. This represents a systematic shift of over 2 {sigma} compared to our earlier estimate, owing completely to the new distance scale{emdash}a shift which we emphasize results not only from the {ital 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 matter-dominated universe (the latter requiring H{sub 0}{le}67kms{sup {minus}1}Mpc{sup {minus}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. {copyright} {ital 1998} {ital The American Astronomical Society}
- OSTI ID:
- 678795
- Journal Information:
- Astrophysical Journal, Vol. 494, Issue 1; Other Information: PBD: Feb 1998
- Country of Publication:
- United States
- Language:
- English
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