Aaron Gellar (CIERA/Northwestern University)

Companions to the NGC 188 Blue Stragglers

The debate over the origin of blue stragglers in open clusters has long suffered from a lack of direct and definitive observational evidence. Correlations between blue straggler frequency and cluster binary fraction, core mass, and radial position suggest that mass transfer or mergers in binaries (or perhaps hierarchical triples) may dominate blue straggler production within open clusters. However, analytic models, detailed observations, and sophisticated N-body simulations argue in favor of stellar collisions. Clearly such indirect evidence is far from conclusive. For blue stragglers in binaries, this debate can be resolved through observational determination of the evolutionary states and masses of blue straggler companion stars. The long-period blue stragglers in the old (7 Gyr) open cluster NGC 188 have companions with masses of about 0.5 Msun, with surprisingly little scatter. This conclusively rules out an origin in collisions, as the collision hypothesis predicts a companion-mass distribution with significantly higher masses. The data are closely consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be 0.5 Msun white dwarfs.