Blue Stragglers in Low-Luminosity Star Clusters

Abstract

We examine the blue straggler populations of 13 low-luminosity (M_Vt>~-6) globular clusters and two old open clusters. These clusters test blue straggler formation in environments intermediate between higher luminosity (and usually higher density) clusters and the Galactic field. The anticorrelation between the relative frequency of blue stragglers (F_BSS=N_BSS/N_HB) and cluster luminosity continues to the lowest luminosity clusters, which have frequencies meeting or exceeding that of field stars. In addition, we find that the anticorrelation between straggler frequency and central density disappears for clusters with density less than about 300 L_V,solar pc^-3, although this appears to be an artifact of the correlation between cluster luminosity and central density. We argue on observational (wide, eccentric binaries containing blue stragglers in M67, and the existence of very bright stragglers in most of the clusters in our sample) and theoretical grounds that stellar collisions still produce a significant fraction of the blue stragglers in low-luminosity star clusters, due to the long-term survival of wide binaries.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.