Stellar evolution in globular clusters

1. Summaries are given of the major stages of evolution for stars in globular clusters, including the nucleosynthesis networks, and their sensitivities to chemical abundances. Difficulties in comparing models with observations are described. 2. Unusual types of stars, including binaries, blue stragglers, and the “second parameter problem” are described. The blue stragglers appear to be the result of mass transfer, and either age or stellar interactions enhanced by high stellar densities are viable explanations for the second parameter, although age appears to be favored on Galactic scales. 3. RR Lyrae variables are discussed, including the relations between M V and [Fe/H] and M K and log P, and how the Oosterhoff classes are defined and differ. Some new results suggest that the two classes might differ in their Galactic histories and not only in metallicity. 4. Stellar populations, including the thick disk and the halo are defined. Possible subcomponents to the halo population are identified, as well as their origins. Part of the halo, that nearer the disk’s plane, may share a common history with the disk, while the halo farther from the plane may be dominated by independent origin(s). The thick disk appears also to have arisen from a merger, based on an overlap in metallicity with the disk but has very different kinematics. 5. A simple model for the metallicity distribution function is described, as well as how metallicities and metallicity indicators are derived and calibrated. Results from recent analyses of high-resolution stellar spectroscopy agree well with each other, but disagree at intermediate metallicities with earlier results. New relations between [Fe/H] and the metallicity indicators ΔS and W′ are derived. 6. Abundances of some special elements are described, including the “α” elements (O, Mg, Si, Ca, Ti) and the s-process and r-process elements, all of which are useful in the study of the Galaxy’s nucleosynthesis history. Halo stars with unusual [α/Fe] ratios are identified and discussed. Unusual ratios for thick disk stars also suggest an origin separate from that of the disk. Lithium abundances in halo stars are also discussed briefly in terms of whether or not halo stars may be used to infer Big Bang nucleosynthesis abundances. Some new evidence for lithium production in low mass stars is presented. 7. Evidence for deep mixing in red giants is presented, including from the CN, ON, NaNe, and MgAl cycles associated with shell hydrogen burning. New evidence is also presented that indicates that helium may also be mixed into red giant photospheres. The possible role of rotation on mixing, mass loss, and horizontal branch morphology is summarized. Evidence for primordial variations in some elements within globular clusters is described, including CN variations in stars on cluster main sequences, and variable iron and r-process abundances in some cluster red giants. 8. The two methods for estimation of relative ages of globular clusters are reviewed: the turn-off luminosity derived from an assumed relation between horizontal branch and metallicity; and the color difference between the main sequence turn-off and the red giant branch at nearly equal metallicities. The most metal-poor globular clusters and field stars seem to show no discernible age differences, but an age spread begins to appear at intermediate metallicities, including in the outer halo. [O/Fe] and [α/Fe] ratios are compared with the relative age scales, suggesting that either some sub-populations (the disk clusters, for example) did not share a common chemical history with the oldest clusters, or that the SNe la timescale may be longer than 1010 years. (Or, as has been suggested recently, that the initial appearance of SNe Ia has more to do with metallicity than with age.) 9. The absolute ages of globular clusters are discussed in terms of the remaining uncertainties. Primary among them is the globular cluster distance scale and the apparent dichotomy from Hipparcos (and other) results for the luminosities of RR Lyrae variables. Differences between field and cluster RR Lyraes may be part of the cause, but the explanation fails at least two tests. Nonetheless, the ages of the globular clusters agree, at least within the large error bars, with the ages of field and cluster white dwarfs derived from deep luminosity functions and cooling theory, and from radioactive dating techniques.