Where the Blue Stragglers Roam: Searching for a Link between Formation and Environment

Abstract

The formation of blue stragglers is still not completely understood,\nparticularly the relationship between formation environment and mechanism. We\nuse a large, homogeneous sample of blue stragglers in the cores of 57 globular\nclusters to investigate the relationships between blue straggler populations\nand their environments. We use a consistent definition of "blue straggler"\nbased on position in the color-magnitude diagram, and normalize the population\nrelative to the number of red giant branch stars in the core. We find that the\npreviously determined anti-correlation between blue straggler frequency and\ntotal cluster mass is present in the purely core population. We find some weak\ncorrelations with central velocity dispersion and with half-mass relaxation\ntime. The blue straggler frequency does not show any trend with any other\ncluster parameter. Even though collisions may be expected to be a dominant blue\nstraggler formation process in globular cluster cores, we find no correlation\nbetween the frequency of blue stragglers and the collision rate in the core. We\nalso investigated the blue straggler luminosity function shape, and found no\nrelationship between any cluster parameter and the distribution of blue\nstragglers in the color-magnitude diagram. Our results are inconsistent with\nsome recent models of blue straggler formation that include collisional\nformation mechanisms, and may suggest that almost all observed blue stragglers\nare formed in binary systems.\n