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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[78.13] Composition Mixing during Blue Straggler Fromation and Evolution

E. Sandquist (Northwestern U), M. Bolte, L. Hernquist (UCSC)

We use smoothed-particle hydrodynamics to examine differences between direct collisions of single stars and binary star mergers in their roles as possible blue straggler star formation mechanisms. From our simulations we find in all cases that core helium in the progenitor stars is largely retained in the core of the remnant, almost independent of the type of interaction or the mass concentration in the progenitor stars.

We have also modelled the subsequent evolution of the hydrostatic remnants, including mass loss and energy input from the hydrodynamical interaction. The combination of the hydrodynamical and hydrostatic models enables us to predict that little mixing will occur during the merger of two globular cluster stars of equal mass. Unmixed and completely mixed models can both explain the luminosities of the brightest blue stragglers by the merger of two turnoff mass stars. In contrast to previous studies, we find that neither completely mixed nor unmixed models can match the absolute colors of observed blue stragglers in NGC 6397 at all luminosity levels. We also find that the color distribution is probably the crucial test for explanations of straggler formation --- if stellar collisions and mergers are the correct mechanisms. a large fraction of the lifetime of the straggler must be spent away from the main sequence. This constraint appears to rule out the possibility of completely mixed models. For NGC 6397, unmixed models predict blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely mixed models predict a range from about 0.6 to 4 Gyr.

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Program listing for Wednesday