What Are These Blue Metal-Poor Stars?

The radial velocity behavior and chemical compositions of sixty-two blue metal-poor (BMP) stars have been established from more than 1200 echelle spectra obtained at Las Campanas Observatory from 1992 through 1999. Analysis of survey spectra provides abundances for this sample, which we use to calibrate 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 CS 22966-043, appears to be the most extreme example of a rare abundance class characterized by α-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 members of binary systems. The binary fraction of BMP stars appears to be independent of chemical composition. The high binary fraction f_BMP~0.6 of BMP stars compared with that 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 blue stragglers and that these blue stragglers are formed by McCrea mass transfer rather than by the various merger processes that are currently believed to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long periods and small orbital eccentricities, properties these binaries share with McClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch evolution. Primaries of more widely separated binaries that survive this phase of stellar evolution may engage in mass transfer during subsequent asymptotic giant branch evolution to form s-process abundance enhanced carbon stars. Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose 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 model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction f_B. Our model calculations return values of S_BS~5 for f_B=0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.