Models of Individual Blue Stragglers

Abstract

As we have seen in previous chapters, blue stragglers are thought to form in both of two broad classes of formation mechanisms: through collisions or through binary mass transfer. Both processes transform two stars into a blue straggler on reasonably short timescales compared to the main sequence lifetimes of low mass stars: a headon collision can take a few days, while stable mass transfer from main sequence stars can take much longer, up to the nuclear timescale of the stars. However, it is not enough to follow the hydrodynamic phases of either process. We must determine the structure of the proto-blue straggler at the moment when the remnant is in hydrostatic equilibrium, and then follow its evolution using a stellar evolution code for the next few billion years in order to make direct comparisons to the observed properties of blue stragglers. Those stellar evolution models are the subject of this chapter.In the context of stellar models, it makes sense to try to define these two formation mechanisms a little more clearly. For our purposes, a collision is a short-lived and strong interaction between two stars. The situation where two previously unrelated stars move through a cluster and find themselves close enough that their radii overlap is obviously a collision, and those could be either head-on or off-axis. A similar situation during a resonant encounter involving binaries or triples can also be modelled using the same techniques as a collision. In this case the two stars might have originally been a binary, but the event that made the blue straggler was not a slow spiral-in of the two stars, but a more dramatic event caused by the overall interaction of the systems. In the literature, these kinds of collisions are often called "binary-mediated". The key distinction that I would like to make is that the event which triggered the hydrodynamic phase of a collision is very short-lived, and de-

Variable Blue Straggler Stars in the Open Cluster NGC 6819 Observed in the Kepler “Superstamp” Field

“…Many researchers have explored blue straggler formation scenarios, including mass transfer from a companion or binary merger, without consideration of asteroseismic constraints. Sills (2015) introduces methods to model blue straggler evolution and discusses the success of various approaches in describing observations. Sandquist et al (1997) use a smooth particle hydrodynamics code to compare the properties of blue stragglers formed by direct collision with those resulting from binary merger and examine the subsequent evolution of the merger products, including mass loss.…”