Some Integrated Properties of Galactic Globular Clusters

Abstract

The luminosities of globular clusters are found to correlate with their half-light radii. The most luminous clusters have half-light radii r ~ 3 pc. Mean cluster luminosities are < Mv > = -6.64 +/- 0.26 for r < 2.0, < Mv > = -7.44 +/- 0.20 for 2.0 < r < 4.0 pc, and < Mv > = -6.57 +/- 0.21 for clusters with r > 4.0 pc. An even fainter value < Mv > = -5.85 +/- 0.36 is found for large clusters with r > 8.0 pc. These results possibly weaken confidence in the conclusion that the peak of the globular cluster luminosity distribution is a universal standard candle. In the outer Galactic halo globular clusters with red horizontal branches are fainter by about a factor of ten than are clusters with blue and intermediate color horizontal branches. Among clusters with Galactocentric distances > 10 kpc there appears to be a clear dichotomy between normal clusters (which all have [Fe/H] < -1.2) and the anomalous relatively metal-rich clusters Pal 1, Pal 12 and Ter 7, which are both unusually faint (Mv > -5) and relatively metal-rich ([Fe/H] > - 1.0). This suggests that these relatively metal-rich halo clusters may have had an unusual evolutionary history.