OBSERVED UPPER LIMITS ON LITHIUM ABUNDANCES IN BLUE STRAGGLERS

High-dispersion echelle spectra which include the Li I lambda-6708 line are reported for four blue stragglers. Two are field Population II stars with [Fe/H] near -1.4, while the other two are members of the open cluster M 67. The Li I line is not detected in the spectra of any of these four blue stragglers; consequent upper limits on the surface lithium abundances are further deduced from suitable model atmospheres. The upper limits are significantly lower than the initial lithium abundances inferred for the respective stellar populations associated with these blue stragglers. The negative results for the two metal-deficient stars may reveal the existence of a narrow "lithium gap" along the Population II main sequence, which is analogous to that seen in the Hyades at spectral types near F5 V. Alternatively, some mechanism associated with the formation and evolution of halo blue stagglers may account for their reduced atmospheric lithium abundance. The negative results in M 67 argue that blue stragglers do not result from delayed star formation. They also suggest that blue stragglers do not remain near the main sequence for extended lifetimes as a result of interior mixing, unless the surfaces of such stars are also mixed. On the other hand, the negative results are to be expected if blue stragglers result from mass transfer or coalescence in binary systems or from stellar collisions.