Flare Production of $^6$Li in Population~II Stars

Abstract

Motivated by the recent report of a $^6$Li detection in the atmosphere of HD~84937, we couple stellar evolution calculations with light isotope production via stellar flares. We find that as a consequence of their small convective envelope mass near the turn-off point, low-metallicity dwarfs and subgiants may possess observable amounts of $^6$Li arising from such flare activity. We point out an observational test which could discriminate between flare produced $^6$Li and protostellar $^6$Li. In the $T_{\rm eff}$ range $6000-6600$~K, the $^7$Li/$^6$Li ratio on the subgiant branch should increase as a function of $T_{\rm eff}$ if flare production is important, whereas the same ratio should be constant if a protostellar origin is the source of the observed lithium. The absence of a flare produced variation in the $^7$Li/$^6$Li ratio would allow for a more reliable inference of the -- cosmologically important -- atmospheric depletion mechanisms in stars.