Stellar Encounters With Multiple Star Systems and the Blue Straggler Problem

Nathan Leigh, McMaster? University

Blue stragglers (BSs) are stars that appear as an extension of the main-sequence (MS) turn-off in cluster color-magnitude diagrams. Simulations have shown that BSs can be formed via the addition of fresh hydrogen to the cores of normal MS stars. Recent observations suggest that several BS formation mechanisms could be operating in clusters simultaneously, including mergers of MS stars and mass-transfer from an evolved donor onto a MS binary companion.

Over the past decade, spectroscopic studies of old open clusters have identified a number of binary and triple systems containing BSs. Many of these are suspected to have formed from the mergers of one or more MS stars during dynamical interactions involving binary stars. In this talk, we present an analytic technique to identify the most probable dynamical formation scenario for an observed binary or triple system containing one or more blue stragglers or, alternatively, to rule out the possibility of a dynamical origin. We apply our analytic prescription to observed binary and triple systems containing BSs, in particular the triple system S1082 in M67 which contains two BSs and the curiously bi-modal period distribution of the BS binaries in NGC 188.