Rapid rotation of ultra-Li-depleted halo stars and their association with blue stragglers

Observations of eighteen halo main-sequence-turnoff stars, four of which are extremely deficient in Li, show that three of the Li-poor ones have substantial line broadening. We attribute this to stellar rotation. Despite the great ages of halo stars, for G202-65, BD+51:1817 and Wolf 550 we infer vsini = 8.3 +/- 0.4, 7.6 +/- 0.3, and 5.5 +/- 0.6 km/s respectively. The stated errors are 3sigma. For CD-31:19466 we derive a 3sigma upper limit vsini < 2.2 km/s. The three rotating stars are known spectroscopic binaries. We explain the high rotation velocities in terms of mass and angular momentum transfer onto the surface of the turnoff star from an initially more-massive donor. Estimates of the specific angular momentum of accreted material indicate that quite small transfer masses could have been involved, though the unknown subsequent spin-down of the accretor prevents us from assigning definitive values for each star. The accretor is now seen as an ultra-Li-deficient star whose origin makes it a low-mass counterpart of field blue stragglers. The Li could have been destroyed before or during the mass-transfer episode. Such objects must be avoided in studies of the primordial Li abundance and in investigations into the way normal single stars process their initial Li.