High lithium-7 ( \mathrm { { } ^ { 7 } Li } ) abundances in giants are indicative of non-standard physical processes affecting the star .
Mechanisms that could produce this signature include contamination from an external source , such as planets , or internal production and subsequent mixing to the stellar surface .
However , distinguishing between different families of solutions has proven challenging , and there is no current consensus model that explains all the data .
The lithium-6 ( \mathrm { { } ^ { 6 } Li } ) abundance may be a potentially important discriminant , as the relative \mathrm { { } ^ { 6 } Li } and \mathrm { { } ^ { 7 } Li } abundances are expected to be different if the enrichment were to come from internal production or from engulfment .
In this work , we model the \mathrm { { } ^ { 6 } Li } and \mathrm { { } ^ { 7 } Li } abundances of different giants after the engulfment of a substellar mass companion .
Given that \mathrm { { } ^ { 6 } Li } is more strongly affected by Galactic chemical evolution than \mathrm { { } ^ { 7 } Li } , \mathrm { { } ^ { 6 } Li } is not a good discriminant at low metallicities , where it is expected to be low in both star and planet .
For modeled metallicities ( [ Fe/H ] > -0.5 ) , we use a “ best case ” initial \mathrm { { } ^ { 6 } Li / ^ { 7 } Li } ratio equal to the solar value . \mathrm { { } ^ { 6 } Li } increases significantly after the engulfment of a companion .
However , at metallicities close to solar and higher , the \mathrm { { } ^ { 6 } Li } signal does not last long in the stellar surface .
As such , detection of surface \mathrm { { } ^ { 6 } Li } in metal-rich red giants would most likely indicate the action of a mechanism for \mathrm { { } ^ { 6 } Li } -enrichment other than planet engulfment .
At the same time , \mathrm { { } ^ { 6 } Li } should not be used to reject the hypothesis of engulfment in a \mathrm { { } ^ { 7 } Li } -enriched giant or to support a particular \mathrm { { } ^ { 7 } Li } -enhancement mechanism .