The Rotational Rates of Blue Stragglers Produced by Physical Stellar Collisions

Blue stragglers produced from physical stellar collisions are expected to be born rotating rapidly, while the blue stragglers observed in intermediate-age and old star clusters do not. We suggest that the collision and merger of two main-sequence stars results in an object that quickly swells into a pre-main-sequence-like star with a convective envelope, which can spin down via magnetic braking as it approaches its thermal equilibrium configuration. The result is a blue straggler with a rotation similar to that of a normal star of the same mass and age. Consequently, rapid rotation is not a signature of a collisionally merged blue straggler.