Metal-poor field blue stragglers: More evidence for mass transfer

We report radial velocity studies of five candidate metal-poor field blue stragglers, all known to be deficient in lithium. Four of the five stars are single-lined spectroscopic binaries, with periods ranging from 302 to 840 days, and low orbital eccentricities, in agreement with similar behavior found for other blue straggler candidates by Preston & Sneden and Carney et al. The limited data available for lithium abundances indicate that all blue straggler binaries have depleted lithium, but that constant velocity stars generally have normal lithium abundances. This suggests that the "lithium gap'' for hot metal-poor main-sequence stars may not exist or lies at higher temperatures than found in the Hyades. Our results and those of Preston & Sneden show higher values of v(rot) sin i for the binary stars than those of comparable temperature constant velocity stars. The orbital periods are too long for tidal effects to be important, implying that spin-up during mass transfer when the orbital separations and periods were smaller is the cause of the enhanced rotation. The mass function distribution is steeper for the blue straggler binary stars than that of lower mass single-lined spectroscopic binaries, indicating a narrower range in secondary masses. We argue that if all secondaries are white dwarfs with the same mass, it is probably around 0.55 M-circle dot. The models of Rappaport et al., applied to white dwarf secondaries, suggest that the orbital elements of all metal-poor binary blue stragglers are consistent with stable mass transfer, with the possible exception of G202-65.