The Long-Period Binary Frequency of the M67 Blue Stragglers and Binary-Binary Collisions

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Session 24 -- Star Clusters/General Relativity
Oral presentation, Wednesday, January 12, 10:15-11:45, Salon V Room (Crystal Gateway)

[24.03] The Long-Period Binary Frequency of the M67 Blue Stragglers and Binary-Binary Collisions

P.J.T. Leonard (LANL)

The old open cluster M67 contains a dozen blue stragglers (BSs), one of which is a short-period spectroscopic binary, and thus is likely the result of binary mass transfer. A clue to the origin of the other BSs may be the fact that ${}^>_\sim~50\%$ of them appear to be members of long-period ($>10^3$ days) binary systems (Milone et al.\ 1991, ASP Conference Series, 13, 424). If the majority of the M67 BSs are due to the slow coalescence of isolated binaries (Mateo et al.\ 1990, AJ, 100, 469), then there are two possible explanations for their anomalously high long-period binary frequency: 1) the frequency of triple star systems in M67 was initially similar to the binary frequency, and the inner components of some of these triple systems have merged to form BSs, or 2) the typical massive star in the core of M67 has suffered an exchange interaction with a binary star. The former solution requires a triple frequency that is vastly higher than in any other stellar population, and thus appears unlikely. The latter requires a rate of interactions involving binary stars in M67 so high that at least some of the BSs in the cluster must be the result of physical stellar collisions during binary-binary interactions. Consequently, one cannot accept the slow binary coalescence scenario for the M67 BSs without accepting that at least some of the M67 BSs have been produced via physical stellar collisions. Of course, the high long-period binary frequency of the M67 BSs can be naturally accounted for by the collisional hypothesis, since the majority of the merged stars produced by binary-binary collisions are expected to possess such companions (Leonard \& Fahlman 1991, AJ, 102, 994; Leonard \& Linnell 1992, AJ, 103, 1928). The high binary frequency observed in M67 (e.g., Montgomery et al.\ 1993, AJ, 106, 181) makes binary-binary interactions inevitable, and thus the collisional hypothesis appears to be quite a realistic possibility.

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