Is Betelgeuse the Outcome of a Past Merger?

We explore the possibility that the star α Orionis (Betelgeuse) is the outcome of a merger that occurred in a low mass ratio (q = M2 /M1 = 0.07-0.25) binary stellar system some time in the past hundreds of thousands of years. To that goal, we present a simple analytical model to approximate the perturbed internal structure of a post-merger object following the coalescence of a secondary in the mass range 1-4 Msun into the envelope of a 15-17 Msun primary. We then compute the long-term evolution of post-merger objects for a grid of initial conditions and make predictions about the luminosity, effective temperature and surface equatorial rotational velocity for evolutionary stages that are consistent with the observed location of Betelgeuse in the Hertzsprung-Russell diagram. We find that if a merger occurred after the end of the primary's main-sequence phase, while it was expanding toward becoming a red supergiant star and with radius 200-500 Rsun, then it's envelope is spun-up to values which remain in a range consistent with the Betelgeuse observations for thousands of years of evolution. We also show that the only scenario that can explain both the fast rotation of Betelgeuse and it's observed large space velocity is one where a binary with q = 0.07-0.25 was dynamically ejected by the parent cluster a few million years ago and then subsequently merged. Our results suggest that a fraction of massive stars with uncharacteristically large rotational velocities during their post-main sequence evolution can be well-modeled by invoking a past merger.