The Relative Ages of Galactic Globular Clusters

We present a review of the present state of knowledge regarding the relative ages of Galactic globular clusters. First, we discuss the relevant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution of the globular clusters. Next, the techniques used to estimate globular cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these techniques as a foundation, we present arguments in favor of the following assertions: 1) The age of a globular cluster is the likeliest candidate to be the global second parameter, which along with metal abundance, controls the morphology of the horizonal branch. 2) A total age range of as much as $\sim$5 Gyr exists among the bulk of the Galactic globulars. 3) There is a significant relation between age and metallicity among the Galactic globular clusters if the slope of the \mvrr-\feh relation is less than $\sim$0.23. These conclusions along with other supporting evidence favor a formation scenario in which the inner regions of the Galactic halo collapsed in a monotonic fashion over a short time period much less than 1 Gyr. In contrast, the outer regions of the halo fragmented and collapsed in a chaotic manner over several Gyrs.Comment: Invited review to appear in PASP. Uses aastex. 45 pages with 9 figure

The Y^2 Stellar Evolutionary Tracks

We present a database of the latest stellar models of the $Y^2$ (Yonsei-Yale) collaboration. This database contains the stellar evolutionary tracks from the pre-main-sequence birthline to the helium core flash that were used to construct the $Y^2$ isochrones. We also provide a simple interpolation routine that generates stellar tracks for given sets of parameters (metallicity, mass, and $\alpha$-enhancement).Comment: 7 pages, TeX, 1 eps figure. ApJS, 2003, vol.144 (Feb), in pres

The hot stars in orbit around the M31 central supermassive black hole: are they young or old?

AIMS: The cluster of hot stars observed in orbit around the central black hole of M31 has been interpreted as a 200 Myr starburst. The formation of a population of young stars in close proximity to a massive black hole presents a difficult challenge to star formation theory. We point out that in a high stellar density environment, the course of stellar evolution is modified by frequent collisions and mergers. METHODS: Blue stragglers, which are the results of mergers in globular clusters, occupy the same position in the color-magnitude diagram as the observed hot stars in M31. For confirmation, the integrated spectrum of P3 is shown to be compatible with the spectral energy distribution of a blue horizontal branch field star. RESULTS: We suggest an old stellar population of evolved blue horizontal-branch stars and of merger products cannot be ruled out on the basis of the available data. Observations are suggested that would help distinguish between a ``young'' and ``old'' stellar population interpretation of the observations.Comment: 8 pages, 2 figures, accepted by A&A for publication, in pres