THE LITHIUM ABUNDANCE OF M67-BLUE STRAGGLERS - A CONSTRAINT ON THE BLUE STRAGGLER PHENOMENON

Upper limits have been placed on the line strength of the 6707 angstrom Li I resonance doublet in seven blue stragglers in M67. The corresponding upper limits on abundances range from log N(Li) less-than-or-similar-to 1.3 to less-than-or-similar-to 2.3. This result is significantly below the level of log N(Li) almost-equal-to 3.1 +/- 0.1 found in field main-sequence stars of comparable temperature. We conclude that some form of mixing has affected the outer envelopes of blue stragglers. (Such mixing has been proposed as the mechanism needed to prolong the lifetimes of blue stragglers relative to normal main-sequence stars at the same luminosity.) Virtually all mechanisms for the production of blue stragglers other than mixing, binary mass transfer, or binary coalescence appear to be ruled out by the present observations.