Blue Straggler Surface Abundances: A Clue to Their Formation History?

Blue stragglers have been a puzzle since they were first discovered. It has become clear that in order to form them multiple stars need to interact. The two most common formation mechanisms involve stellar collisions and binary mass transfer. It has been proposed that these two formation channels can be distinguished by looking at the surface abundances of the blue straggler. However, there have been no detailed comparison of the expected abundance pattern for these two scenarios from detailed stellar models. We have calculated blue straggler models from stellar collisions and from binary mass transfer, and compared their predicted surface abundances. We find that it is not straightforward to distinguish blue stragglers that formed through mass transfer from blue stragglers that formed as a result of a stellar collision based on CNO elements, however it may be possible to use Li as a tracer instead. In addition, it may be possible to estimate the time since formation of the blue straggler based on the abundance of heavy elements that are affected by radiative levitation.