Blue Stragglers and Friends: Initial Evolutionary Pathways in Close Low-Mass Binaries

The scope of this review is the first stage in the evolution of close binary stars having components with M < 2 M ⊙ . An observational taxonomy for the products of such binary evolution is provided in the framework of dwarfs (blue straggler stars), giants (yellow straggler stars), subdwarf B stars, and giant-like stars (sub-subgiant stars and red straggler stars). ▪ Blue stragglers and yellow stragglers have directly measured masses greater than the main sequence turnoff masses of coeval populations. Observational evidence points to mass transfer as the most frequent formation channel for first-stage binary evolution products, occurring with enhanced stability and a range of mass-transfer efficiencies. ▪ Rapid rotation is an observed hallmark of products and an expected outcome of all proposed formation channels—mass transfer, mergers, and collisions. Excess angular momentum must be removed to permit observed mass gains by processes yet to be understood. ▪ Key theoretical issues remain. The stability of mass transfer from red giant and asymptotic giant branch donor stars remains ill-understood. Models struggle to account for the observed distributions of orbital eccentricities and periods. The loss of mass and angular momentum from a binary system is largely unconstrained. Detailed physical models for mergers of low-mass main sequence binaries are lacking. First-stage binary evolution products constitute a substantial fraction of all evolved stars in old stellar populations. They travel along major alternative pathways of stellar evolution and in regions of the Hertzsprung–Russell diagram not populated by single stars.