SIMBAD references

Evolutionary tracks and isochrones were calculated with alpha-enhanced compositions which cover the entire globular cluster metallicity range and include the effects of the diffusion of ^4He. Including the effects of helium diffusion has a negligible effect (< 0.5 Gyr) on the derived ages of globular clusters. Regardless of the inclusion of helium diffusion, a significant age spread of ∼5 Gyr exists among the globular clusters. The oldest globular cluster studied was M92 with an age of 17 ± 2 Gyr old.

The stellar models may be tested by comparing the Li depletion and surface rotation rates to observations in young clusters stars. The observed Li abundances clearly indicate that standard or diffusive models do not deplete enough Li. Instabilities induced by rotation provide an additional mixing mechanism. For this reason the stellar evolution code was modified to include the combined effects of diffusion and rotational mixing on ^1H, 4He and the trace elements ^3He, ^6Li, ^7Li, and ^9Be. The calibrated solar models have a convection zone depth of 0.709 - 0.714∼R_odot, in excellent agreement with the observed depth of (0.713 ± 0.003)∼R_odot. The rotational mixing inhibits the diffusion in the outer parts of the models, leading to a decrease in the envelope diffusion by 30 - 50%. The combined models are able to simultaneously match the Li abundances observed in the Pleiades, UMaG, Hyades, NGC 752 and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to explain the presence of the rapidly rotating G and K stars in the Pleiades.

Observations of Li abundances in extremely metal poor halo stars provide another test of the stellar models. All models which use Kurucz (1992) model atmospheres to determine the surface boundary conditions are unable to match the observed Li depletion in cool halo stars. Models which use the gray atmosphere approximation provide a much better fit to the data. Standard models do a good job of fitting the observed Li abundances and predict a primordial Li abundance of log N(Li) = 2.24 ± 0.03. In models which include microscopic diffusion, but not rotational mixing, too much Li depletion occurs in the hot stars, and the models do not match the observations. The [Fe/H] = -2.28 stellar models which include both diffusion and rotational mixing provide an excellent match to the observations and predict a primordial Li abundance of log N(Li) = 3.13 ± 0.1. However, the [Fe/H] = -3.28 Li destruction isochrones reveal problems with the combined models. These low metallicity isochrones predict a trend of decreasing log N(Li) with increasing effective temperature which is clearly not present in the observations.