The Age of Globular Clusters

I review here recent developments which have affected our understanding of both the absolute age of globular clusters and the uncertainties in this age estimate, and comment on the implications for cosmological models. This present estimate is in agreement with the range long advocated by David Schramm. The major uncertainty in determining ages of globular clusers based upon the absolute magnitude of the main sequence turn-off remains the uncertainty in the distance to these clusters. Estimates of these distances have recently been upwardly revised due to Hipparcos parallax measurements, if one calibrates luminosities of main sequence stars. However, it is important to realize that at the present time, different distance measures are in disagreement. A recent estimate is that the oldest clusters are $11.5 \pm 1.3$ Gyr, implying a one-sided 95% confidence level lower limit of 9.5 Gyr, if statistical parallax distance measures are not incorporated. Incorporating more recent measures, including Hipparcos based statistical parallax measures, raises the mean predicted age to $12.8 \pm 1$ Gyr, with a 95 % confidence range of 10-17 Gyr. I conclude by discussing possible improvements which may allow a more precise age distribution in the near future.Comment: latex (using elsart macro for Physics Reports), 16 pages including 4 figures. To appear in Physics Reports, David Schramm Memorial Volum

The Primordial Abundance of 6^6Li and 9^9be

Light element ($^6$Li, $^7$Li and $^9$Be) depletion isochrones for halo stars have been calculated with standard stellar evolution models. These models include the latest available opacities and are computed through the sub-giant branch. If $^6$Li is not produced in appreciable amounts by stellar flares, then the detection of $^6$Li in HD 84937 by Smith, Lambert \& Nissen (1993) is compatible with standard stellar evolution and standard big bang nucleosynthesis only if HD 84937 is a sub-giant. The present parallax is inconsistent with HD 84937 being a sub-giant star at the $2.5\, \sigma$ level. The most metal poor star with a measured $^9$Be abundance is HD 140283, which is a relatively cool sub-giant. Standard stellar evolution predict that $^9$Be will have been depleted in this star by $\sim 0.3$ dex (for ${\rm T_{eff}} = 5640$ K). Revising the abundance upward changes the oxygen-beryllium relation, suggesting incompatible with standard comic ray production models, and hence, standard big bang nucleosynthesis. However, an increase in the derived temperature of HD 140283 to 5740 K would result in no depletion of $^9$Be and agreement with standard big bang nucleosynthesis.Comment: 6 pages, AAS LaTeX, complete postscript file available via anonymous ftp from: ftp.cita.utoronto.ca in /cita/brian/papers/primord.p

Investigating the Consistency of Stellar Evolution Models with Globular Cluster Observations via the Red Giant Branch Bump

Synthetic RGBB magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced $\alpha$ capture [$\alpha$/Fe] $=+0.4$ and an age of 13 Gyr shows agreement with observations over metallicities ranging from [Fe/H] = $0$ to [Fe/H] $\approx-1.5$, with discrepancy emerging at lower metallicities.Comment: 11 pages, 12 figure

The OPAL Equation of State and Low Metallicity Isochrones

The Yale stellar evolution code has been modified to use the OPAL equation of state tables (Rogers 1994). Stellar models and isochrones were constructed for low metallicity systems ($-2.8 \le [Fe/H] \le -0.6$). Above M\sim 0.7\,\msun, the isochrones are very similar to those which are constructed using an equation of state which includes the analytical Debye-Huckel correction at high temperatures. The absolute magnitude of the main sequence turn-off (\mvto) with the OPAL or Debye-Huckel isochrones is about 0.06 magnitudes fainter, at a given age, than \mvto derived from isochrones which do not include the Debye-Huckel correction. As a consequence, globular clusters ages derived using \mvto are reduced by 6 -- 7\% as compared to the ages determined from the standard isochrones. Below M\sim 0.7\,\msun, the OPAL isochrones are systematically hotter (by approximately 0.04 in B-V) at a given magnitude as compared to the standard, or Debye-Huckel isochrones. However, the lower mass models fall out of the OPAL table range, and this could be the cause of the differences in the location of the lower main-sequences.Comment: to appear in ApJ, 8 pages LaTeX, uses aaspptwo.sty. Complete uuencoded postscript file (including figures) available from: ftp://ftp.cita.utoronto.ca/cita/chaboyer/papers/opal.u

Self-Consistent Magnetic Stellar Evolution Models of the Detached, Solar-Type Eclipsing Binary EF Aquarii

We introduce a new one-dimensional stellar evolution code, based on the existing Dartmouth code, that self-consistently accounts for the presence of a globally pervasive magnetic field. The methods involved in perturbing the equations of stellar structure, the equation of state, and the mixing-length theory of convection are presented and discussed. As a first test of the code's viability, stellar evolution models are computed for the components of a solar-type, detached eclipsing binary (DEB) system, EF Aquarii, shown to exhibit large disagreements with stellar models. The addition of the magnetic perturbation corrects the radius and effective temperature discrepancies observed in EF Aquarii. Furthermore, the required magnetic field strength at the model photosphere is within a factor of two of the magnetic field strengths estimated from the stellar X-ray luminosities measured by ROSAT and those predicted from Ca II K line core emission. These models provide firm evidence that the suppression of thermal convection arising from the presence of a magnetic field is sufficient to significantly alter the structure of solar-type stars, producing noticeably inflated radii and cooler effective temperatures. The inclusion of magnetic effects within a stellar evolution model has a wide range of applications, from DEBs and exoplanet host stars to the donor stars of cataclysmic variables.Comment: Accepted for publication in ApJ, 15 pages, 3 figures; Misprints are corrected in version