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 angul ar momentum loss cannot be the most important sources of blue straggle rs in M67, since the exchange of blue stragglers, or their close binar y progenitors, into detectable binaries that remain bound to the clust er is too inefficient to explain the observed frequency of companions. Case B mass transfer can only account for F190, the one blue straggle r 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 pert urbations from passing stars are not effective enough to yield eccentr ic 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 ar e naturally accounted for. However, the blue stragglers with companion s in orbits consistent with being circular are unlikely to be the resu lt of collisions. Therefore, it appears probable that several mechanis ms have been at work in M67 to have created a blue straggler populatio n with the observed diversity in binary properties.