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) bluestraggler stars (BSS), as inferred from our new catalog containing nearly 3000BSS. The catalog has been extracted from the photometrically homogeneous V vs.(B-V) color-magnitude diagrams (CMD) of 56 GCs, based on WFPC2 images of theircentral cores. In our analysis we used consistent relative distances based onthe same photometry and calibration. The number of BSS has been normalized toobtain relative frequencies (F_{BSS}) and specific densities (N_S) usingdifferent stellar populations extracted from the CMD. The cluster F_{BSS} issignificantly smaller than the relative frequency of field BSS. We find asignificant anti-correlation between the BSS relative frequency in a clusterand its total absolute luminosity (mass). There is no statistically significanttrend between the BSS frequency and the expected collision rate. F_{BSS} doesnot depend on other cluster parameters, apart from a mild dependence on thecentral density. PCC clusters act like normal clusters as far as the BSSfrequency is concerned. We also show that the BSS luminosity function for themost luminous clusters is significantly different, with a brighter peak andextending to brighter luminosities than in the less luminous clusters. Theseresults imply that the efficiency of BSS production mechanisms and theirrelative importance vary with the cluster mass.Comment: 12 pages, 3 figures. accepted for publication in ApJ