Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms

We discuss the main properties of the Galactic globular cluster (GC)blue straggler stars (BSSs), as inferred from our new catalog containingnearly 3000 BSSs. The catalog has been extracted from thephotometrically homogeneous V versus (B-V) color-magnitude diagrams(CMDs) of 56 GCs, based on Wide Field Planetary Camera 2 images of theircentral cores. In our analysis, we used consistent relative distancesbased on the same photometry and calibration. The number of BSSs hasbeen normalized to obtain relative frequencies (F<SUB>BSS</SUB>) andspecific densities (N<SUB>S</SUB>) using different stellar populationsextracted from the CMD. The cluster F<SUB>BSS</SUB> is significantlysmaller than the relative frequency of field BSSs. We find a significantanticorrelation between the BSS relative frequency in a cluster and itstotal absolute luminosity (mass). There is no statistically significanttrend between the BSS frequency and the expected collision rate. Thevalue of F<SUB>BSS</SUB> does not depend on other cluster parameters,apart from a mild dependence on the central density. Post-core-collapseclusters act like normal clusters as far as the BSS frequency isconcerned. We also show that the BSS luminosity function for the mostluminous clusters is significantly different, with a brighter peak andextending to brighter luminosities than in the less luminous clusters.These results imply that the efficiency of BSS production mechanisms andtheir relative importance vary with the cluster mass.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.