Field Blue Stragglers and Related Mass Transfer Issues

Abstract

I view field blue stragglers (FBS) through the prism of an aging observer. This chapter contains my impressions and perspectives about the current state of knowledge about FBSSs, drawn from an extensive literature that I searched with the aid of the NASA Astrophysics Data System, and Google. The search prior to 2000 is spotty and I make no claim of completeness. In documenting various topics I tried to use references that acknowledge important earlier investigations.

Extensive N-body modeling of NGC 188 with empirical initial conditions reproduces the properties of the cluster, and in particular the main-sequence solar-type binary population. The current models also reproduce well the binary orbital properties of the blue stragglers, but fall well short of producing the observed number of blue stragglers. This deficit could be resolved by reducing the frequency of common-envelope evolution during Roche lobe overflow. Both the observations and the N-body models strongly indicate that the long-period blue-straggler binaries—which dominate the NGC 188 blue-straggler population—are formed by asymptotic-giant (primarily) and red-giant mass transfer onto main-sequence stars. The models suggest that the few non-velocity-variable blue stragglers formed from mergers or collisions. Several remarkable short-period double-lined binaries point to the importance of subsequent dynamical exchange encounters, and provide at least one example of a likely collisional origin for a blue straggler.