We explore the possibility that the star \alpha Orionis ( Betelgeuse ) is the outcome of a merger that occurred in a low mass ratio ( q = \mathcal { M } _ { 2 } / \mathcal { M } _ { 1 } = 0.07–0.25 ) binary 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 M _ { \odot } into the envelope of a 15–17 M _ { \odot } primary .
We then compute the long–term evolution of post–merger objects for a grid of initial conditions and make predictions about their surface properties 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 typically with radius \sim 200–300 R _ { \odot } , 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 argue that the best scenario that can explain both the fast rotation of Betelgeuse and its observed large space velocity is one where a binary was dynamically ejected by its parent cluster a few million years ago and then subsequently merged .
An alternative scenario in which the progenitor of Betelgeuse was spun up by accretion in a binary and released by the supernova explosion of the companion requires a finely tuned set of conditions but can not be ruled out .