What are these blue metal-poor stars?

The radial velocity behavior and chemical compositions of sixty-two blue me tal-poor (BMP) stars have been established from more than 1200 echelle spec tra obtained at Las Campanas Observatory from 1992 through 1999. Analysis o f survey spectra provides abundances for this sample, which we use to calib rate the K line versus B-V relation. Forty-four of the stars have [Fe/H] < - 1, while eighteen lie on -1 < [Fe/H] < 0. One star, the SX Phe variable C S 22966-043, appears to be the most extreme example of a rare abundance cla ss characterized by alpha-element deficiencies, high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and [Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities, while 42 are definite or probable membe rs of binary systems. The binary fraction of BMP stars appears to be indepe ndent of chemical composition. The high binary fraction f(BMP) similar to 0.6 of BMP stars compared with t hat found for the F- and G-type stars near the Sun, the systematically low mass functions of these binaries, and the paucity of double-lined binaries among them lead us to suggest that at least half of the BMP binaries are bl ue stragglers and that these blue stragglers are formed by McCrea mass tran sfer rather than by the various merger processes that are currently believe d to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long perio ds and small orbital eccentricities, properties these binaries share with M cClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch e volution. Primaries of more widely separated binaries that survive this pha se of stellar evolution may engage in mass transfer during subsequent asymp totic giant branch evolution to form s-process abundance enhanced carbon st ars. Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field st ars. The high original estimate of the size of this component probably aros e from improper use of the globular cluster BS specific frequency, S-BS = n (BS)/n(HB) - 1, to estimate the halo BS space density. We use a simple mode l to calculate the specific frequency of BSs produced by McCrea mass transf er in an old metal-poor population with a given primordial binary fraction f(B). Our model calculations return values of S-BS similar to 5 for f(B) = 0.15, much more like our value for the field blue stragglers. We suggest th at globular clusters either destroy the primordial binaries that produce lo ng period BS binaries like those in the Galactic field reported here, or th ey never possessed them.