Blue Straggler Formation in Open Clusters

Aaron M. Geller (Northwestern University, USA) and Robert Mathieu (University of Wisconsin, USA)
 

Open clusters occupy a compelling regime for blue straggler formation, where their moderate stellar 
densities permit blue straggler formation through all of the known mechanisms, namely collisions, mergers 
and mass transfer.  Furthermore, the accessibility of open clusters to both detailed observations and 
sophisticated N-body simulations provides unique opportunities to investigate the origins of blue straggler 
stars in great detail.  Perhaps the most well-studied blue straggler population in an open cluster is that of 
NGC 188.  This ~7 Gyr open cluster hosts 21 blue straggler stars, 16 of which are known to have binary 
companions, as revealed by our multi-epoch radial-velocity survey.  The majority of these blue straggler stars 
have ~0.5 Msun companions that orbit with periods of order 1000 days and modest eccentricities.  
Comparisons with predictions from N-body simulations of NGC 188 show that collisions and mergers cannot 
create such blue straggler stars. (Although these mechanisms may account for some or all of the other blue 
straggler stars in the cluster.) Instead the dominant formation mechanism for the blue straggler stars in NGC 
188 is most likely mass transfer processes.  Importantly, the NGC 188 blue straggler stars have very similar 
binary characteristics to those of the ~4 Myr old open cluster M67, and the blue straggler stars in the Galactic 
field, which suggests that blue straggler formation may proceed in a similar manner within a variety of 
environments.  Observations of a number of additional open clusters spanning a large range in age are 
currently underway and will soon provide an unprecedented comparison to these few well-studied and 
remarkably similar blue straggler populations.