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

Abstract

We present data for four ultraLi-deficient, warm, halo stars. The Li deficiency of two of these is a new discovery. Three of the four stars have e+ective temperatures K, in contrast to pre- T eff D 6300 viously known Li-deficient halo stars, which spanned the temperature range of the Spite plateau. In this paper we propose that these and previously known ultraLi-deficient halo stars may have had their surface lithium abundances reduced by the same mechanism as produces halo field blue stragglers. Even though these stars have yet to reveal themselves as blue stragglers, they might be regarded as "" bluestragglers -to-be. In our proposed scenario, the surface abundance of Li in these stars could be destroyed (1) during the normal premain-sequence single-star evolution of their low-mass precursors, (2) during the postmain-sequence evolution of an evolved mass donor, and/or (3) via mixing during a masstransfer event or stellar merger. The warmest Li-deficient stars at the turno+ would be regarded as emerging "" canonical blue stragglers, whereas cooler ones represent subturno+-mass blue-stragglers-tobe. The latter are presently hidden on the main sequence, Li depletion being possibly the clearest signature of their past history and future significance. Eventually, the main-sequence turno+ 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 unied view is the observation that the three Li-depleted stars at K are all binaries, whereas very few of the cooler systems show evidence for binarity ; it is T eff ^ 6300 thus possible that two separate mechanisms are responsible for the production of Li-deficient mainsequence halo stars.