The implications of the binary properties of the M67 blue stragglers

The frequency and properties of spectroscopic binaries among the blue stragglers in the old open cluster M67 observed by Milone, Latham, and collaborators are compared with the various hypotheses for the origin of these stars. Case A mass transfer and binary coalescence via angular momentum loss cannot be the most important sources of blue stragglers in M67, since the exchange of blue stragglers, or their close binary progenitors, into detectable binaries that remain bound to the cluster is too inefficient to explain the observed frequency of companions. Case B mass transfer can only account for F190, the one blue straggler in a short-period binary. Case C mass transfer is Likely a source of blue stragglers in M67, but cannot be the dominant source, since perturbations from passing stars are not effective enough to yield eccentric orbits. Physical stellar collisions during binary-binary and binary-single interactions cannot be ruled out as the most important source of blue stragglers in M67, since the companions in eccentric orbits are naturally accounted for. However, the blue stragglers with companions in orbits consistent with being circular are unlikely to be the result of collisions. Therefore, it appears probable that several mechanisms have been at work in M67 to have created a blue straggler population with the observed diversity in binary properties.