Ultra–Lithium‐deficient Halo Stars and Blue Stragglers: A Common Origin?

We present data for four ultra-Li-deficient, warm, halo stars. The Lideficiency of two of these is a new discovery. Three of the four stars haveeffective temperatures Teff ~ 6300 K, in contrast to previously knownLi-deficient halo stars which spanned the temperature range of the SpitePlateau. In this paper we propose that these, and previously knownultra-Li-deficient halo stars, may have had their surface lithium abundancesreduced by the same mechanism as produces halo field blue stragglers. Eventhough these stars have yet to reveal themselves as blue stragglers, they mightbe regarded as "blue-stragglers-to-be." In our proposed scenario, the surfaceabundance of Li in these stars could be destroyed (a) during the normalpre-main-sequence single star evolution of their low mass precursors, (b)during the post-main-sequence evolution of a evolved mass donor, and/or (c) viamixing during a mass-transfer event or stellar merger. The warmest Li-deficientstars at the turnoff would be regarded as emerging "canonical" blue stragglers,whereas cooler ones represent sub-turnoff-mass "blue-stragglers-to-be." Thelatter are presently hidden on the main sequence, Li depletion being possiblythe clearest signature of their past history and future significance.Eventually, the main sequence turnoff will reach down to their mass, exposingthose Li-depleted stars as canonical blue stragglers when normal stars of thatmass evolve away. Arguing against this unified view is the observation that thethree Li-depleted stars at Teff ~ 6300 K are all binaries, whereas very few ofthe cooler systems show evidence for binarity; it is thus possible that twoseparate mechanisms are responsible for the production of Li-deficientmain-sequence halo stars.Comment: 23 pages including 3 figures. 2001, ApJ, 547, xxx (1 February