Ultra-lithium-deficient halo stars and blue stragglers: A common origin?

We present data for four ultra-Li-deficient, warm, halo stars. The Li defic iency of two of these is a new discovery. Three of the four stars have effe ctive temperatures T-eff similar to 6300 K, in contrast to previously known Li-deficient halo stars, which spanned the temperature range of the Spite plateau. In this paper we propose that these and previously known ultra-Li- deficient halo stars may have had their surface lithium abundances reduced by the same mechanism as produces halo field blue stragglers. Even though t hese stars have yet to reveal themselves as blue stragglers, they might be regarded as "blue-stragglers-to-be" In our proposed scenario, the surface a bundance of Li in these stars could be destroyed (1) during the normal pre- main-sequence single-star evolution of their low-mass precursors, (2) durin g the post-main-sequence evolution of an evolved mass donor, and/or (3) via mixing during a mass-transfer event or stellar merger. The warmest Li-defi cient stars at the turnoff would be regarded as emerging "canonical" blue s tragglers, whereas cooler ones represent sub-turnoff-mass blue-stragglers-t o-be. The latter are presently hidden on the main sequence, Li depletion be ing possibly the clearest signature of their past history and future signif icance. Eventually, the main-sequence turnoff will reach down to their mass , exposing those Li-depleted stars as canonical blue stragglers when normal stars of that mass evolve away. Arguing against this unified view is the o bservation that the three Li-depleted stars at T-eff similar or equal to 63 00 K are all binaries, whereas very few of the cooler systems show evidence for binarity; it is thus possible that two separate mechanisms are respons ible for the production of Li-deficient main-sequence halo stars.