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

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

Theoretical Uncertainties in the Subgiant--Mass Age Relation and the Absolute Age of Omega Cen

The theoretical uncertainties in the calibration of the relationship between the subgiant mass and age in metal-poor stars are investigated using a Monte Carlo approach. Assuming that the mass and iron abundance of a subgiant star are known exactly, uncertainties in the input physics used to construct stellar evolution models and isochrones lead to a Gaussian 1-sigma uncertainty of +/-2.9% in the derived ages. The theoretical error budget is dominated by the uncertainties in the calculated opacities. Observations of detached double lined eclipsing binary OGLEGC-17 in the globular cluster Omega Cen have found that the primary is on the subgiant branch with a mass of M = 0.809+/-0.012 M_sun and [Fe/H]= -2.29+/-0.15 (Kaluzny et al. 2001). Combining the theoretical uncertainties with the observational errors leads to an age for OGLEGC-17 of 11.10+/-0.67 Gyr. The one-sided, 95% lower limit to the age of OGLEGC-17 is 10.06 Gyr, while the one-sided, 95% upper limit is 12.27 Gyr.Comment: 4 pages, 3 figures, to appear in ApJ

Magnetic Inhibition of Convection and the Fundamental Properties of Low-Mass Stars. II. Fully Convective Main Sequence Stars

We examine the hypothesis that magnetic fields are inflating the radii of fully convective main sequence stars in detached eclipsing binaries (DEBs). The magnetic Dartmouth stellar evolution code is used to analyze two systems in particular: Kepler-16 and CM Draconis. Magneto-convection is treated assuming stabilization of convection and also by assuming reductions in convective efficiency due to a turbulent dynamo. We find that magnetic stellar models are unable to reproduce the properties of inflated fully convective main sequence stars, unless strong interior magnetic fields in excess of 10 MG are present. Validation of the magnetic field hypothesis given the current generation of magnetic stellar evolution models therefore depends critically on whether the generation and maintenance of strong interior magnetic fields is physically possible. An examination of this requirement is provided. Additionally, an analysis of previous studies invoking the influence of star spots is presented to assess the suggestion that star spots are inflating stars and biasing light curve analyses toward larger radii. From our analysis, we find that there is not yet sufficient evidence to definitively support the hypothesis that magnetic fields are responsible for the observed inflation among fully convective main sequence stars in DEBs.Comment: Accepted for publication in ApJ, 17 pages, 11 figures, 2 table