Is there a Population II analogy to the F star lithium dip?

Observers have found a small number of lithium-depleted halo stars in the temperature range of the Spite plateau. The current status of the mass-loss hypothesis for producing the observed lithium dip in Population (Pop) I stars is briefly discussed and extended to Pop II stars as a possible explanation for these halo objects. Based on detections of F-type main-sequence variables, mass loss is assumed to occur in a narrow temperature region corresponding to this “instability strip.” As Pop II main-sequence stars evolve to the blue, they enter this narrow temperature region, then move back through the lower temperature area of the Spite plateau. If 0.05 M⊙ (solar mass) or more have been lost, they will show lithium depletion. This hypothesis affects the lithium-to- beryllium abundance, the ratio of high- to low-lithium stars, and the luminosity function. Constraints on the mass-loss hypothesis due to these effects are discussed. Finally, mass loss in this temperature range would operate in stars near the turnoff of metal-poor globular clusters, resulting in apparent ages 2 to 3 Gyr (gigayears) older than they actually are.

Globular cluster ages

We review two new methods to determine the age of globular clusters (GCs). These two methods are more accurate than the classical isochrone fitting technique. The first method is based on the morphology of the horizontal branch and is independent of the distance modulus of the globular cluster. The second method uses a careful binning of the stellar luminosity function and determines simultaneously the distance and age of the GC. We find that the oldest galactic GCs have an age of 13.5 ± 2 gigayears (Gyr). The absolute minimum age for the oldest GCs is 10.5 Gyr (with 99% confidence) and the maximum 16.0 Gyr (with 99% confidence). Therefore, an Einstein–De Sitter Universe (Ω = 1) is not totally ruled out if the Hubble constant is about 65 ± 10 Km s−1 Mpc−1.