SIMBAD references

Using CCD color-magnitude diagrams of Galactic globular clusters in the B and V filter bands, we focus on the main sequence turnoff region and its usefulness in studying blue straggler stars, relative cluster ages, and the luminosity of the horizontal branch. In addition, we have measured several basic photometric parameters for the clusters in our program; these include NGC 288, NGC 3201, NGC 4372, NGC 4590 (M68), IC 4499, NGC 5897, NGC 6101, NGC 6171 (M107), NGC 6496, NGC 6584, NGC 6723, NGC 6809 (M55), NGC 6934, and NGC 6981 (M72).

Of the 14 globular clusters in our sample, 11 appear to have definite blue straggler sequences. In the majority of cases, the blue stragglers are found to be more centrally concentrated and therefore more massive than the subgiant branch stars. Furthermore, the characteristics of the clusters which contain blue stragglers span practically the entire observed range for Galactic globular clusters. It therefore seems that all such systems must contain blue stragglers. The HR Diagram of the blue stragglers reveals that all of these stars occupy the same general region of this diagram, but the mean location of the individual cluster sequences do scale with metal abundance in the expected fashion. Finally, the luminosity function indicates the possible identification of blue stragglers which have evolved away from their initial positions.

There exists a range in age among the Galactic globular clusters of 3 to 5 gigayears. Such a range suggests a relatively slow formation timescale for the Galactic halo. In addition, from the variation of age as a function of horizontal branch type, we conclude that age is the 'second parameter' which, along with metallicity, governs the behavior of the horizontal branch.

Finally, we derive a simple algebraic expression that describes the slope of the horizontal branch luminosity - metallicity relation in terms of three relative cluster parameters. Using available values of these quantities in 8 globular clusters, we compute a slope of 0.15 ± 0.05 (mag/dex) for the variation of horizontal branch luminosity with metallicity.